N3/P2: give the combat engine an N-player roster

Splits combatState into a fight-scoped half and a per-character half.
Everything that belongs to one PC -- HP, ward/spore/reflect charges,
heal charges, poison ticks, the death save, Lucky/Rage, the
first-attack one-shots, the arcane ward, concentration, and the
debuffs an enemy stacks onto a specific character -- moves to a new
`actor`. What belongs to the fight stays: the enemy pool, the enemy's
stance (evade/block/advantage/retaliate/regen/survive), the round
counter, the event log, and the RNG stream.

combatState embeds *actor, so the promoted fields keep their names and
all ~230 existing reads (st.playerHP, st.wardCharges, ...) compile
untouched. The embedded pointer is a cursor: seat(i) points it at a
roster member. Solo seats one actor and never moves the cursor, so the
draw order off the single RNG stream is unchanged.

That is the whole point. TestCombatCharacterization -- 57 scenarios x
5 seeds, 7468 pinned golden lines -- is byte-identical before and
after. Solo combat provably did not move, so the d8prereq balance
corpus survives the parties work and only party bands need new
baselines in P7.

Hold-person is fight-scoped (holding the enemy holds it for everyone)
while stat_drain/debuff/max_hp_drain are per-character, which is why
they landed on opposite sides of the split.

No multi-actor *semantics* here: nothing yet decides who the enemy
swings at or how initiative interleaves N players. That is P3. This
commit only lands the data model, and the roster tests cover what the
solo golden structurally cannot see -- cursor isolation, shared-state
visibility across seats, and the pointer embed (a value embed would
silently copy on seat() and fail the round-trip assertion).

internal/plugin/bestiary_srd_test.go

@@ -39,9 +39,23 @@ func TestEnemyAttackProfile_Registered(t *testing.T) {
 	}
 }
 
+// testCombatState seats a single actor, the shape every direct-primitive test
+// wants. The engine reads per-actor state through the embedded cursor, so a
+// combatState with a nil actor panics on the first st.playerHP touch.
+func testCombatState(playerHP, enemyHP, round int, rng *rand.Rand) *combatState {
+	a := &actor{playerHP: playerHP}
+	return &combatState{
+		actor:   a,
+		actors:  []*actor{a},
+		enemyHP: enemyHP,
+		round:   round,
+		rng:     rng,
+	}
+}
+
 func TestTurnAbilityFires(t *testing.T) {
 	enemy := baseEnemy() // MaxHP 60
-	st := &combatState{rng: rand.New(rand.NewPCG(1, 1))}
+	st := testCombatState(0, 0, 0, rand.New(rand.NewPCG(1, 1)))
 
 	cases := []struct {
 		name     string
@@ -78,10 +92,7 @@ func TestTurnAbilityFires(t *testing.T) {
 // within applyAbility with no persistent state.
 func TestApplyAbility_Slice2Effects(t *testing.T) {
 	newState := func(playerHP, enemyHP int) (*combatState, Combatant, Combatant) {
-		st := &combatState{
-			playerHP: playerHP, enemyHP: enemyHP, round: 1,
-			rng: rand.New(rand.NewPCG(7, 7)),
-		}
+		st := testCombatState(playerHP, enemyHP, 1, rand.New(rand.NewPCG(7, 7)))
 		return st, basePlayer(), baseEnemy()
 	}
 	phase := &turnCombatPhase
@@ -136,10 +147,7 @@ func TestApplyAbility_Slice2Effects(t *testing.T) {
 // all but evade), and the shared resolution primitives read that state.
 func TestApplyAbility_Slice3Effects(t *testing.T) {
 	newState := func(playerHP, enemyHP int) (*combatState, Combatant, Combatant) {
-		st := &combatState{
-			playerHP: playerHP, enemyHP: enemyHP, round: 1,
-			rng: rand.New(rand.NewPCG(9, 9)),
-		}
+		st := testCombatState(playerHP, enemyHP, 1, rand.New(rand.NewPCG(9, 9)))
 		return st, basePlayer(), baseEnemy()
 	}
 	phase := &turnCombatPhase
@@ -188,7 +196,8 @@ func TestApplyAbility_Slice3Effects(t *testing.T) {
 	}
 
 	// enemyDown lets survive_at_1 cheat death exactly once.
-	stS := &combatState{enemyHP: 0, enemySurviveArmed: true}
+	stS := testCombatState(100, 0, 1, rand.New(rand.NewPCG(13, 13)))
+	stS.enemySurviveArmed = true
 	if enemyDown(stS, "Duel") {
 		t.Error("survive_at_1: armed enemy at 0 HP should not be down")
 	}
@@ -224,10 +233,7 @@ func TestApplyAbility_Slice3Effects(t *testing.T) {
 // itself, and the shared resolution primitives / helpers read that state.
 func TestApplyAbility_Slice4Effects(t *testing.T) {
 	newState := func(playerHP, enemyHP int) (*combatState, Combatant, Combatant) {
-		st := &combatState{
-			playerHP: playerHP, enemyHP: enemyHP, round: 1,
-			rng: rand.New(rand.NewPCG(11, 11)),
-		}
+		st := testCombatState(playerHP, enemyHP, 1, rand.New(rand.NewPCG(11, 11)))
 		return st, basePlayer(), baseEnemy()
 	}
 	phase := &turnCombatPhase

internal/plugin/combat_engine.go

@@ -292,10 +292,23 @@ var bossCombatPhases = []CombatPhase{
 
 // ── Simulation ───────────────────────────────────────────────────────────────
 
-// combatState tracks mutable state during the simulation.
-type combatState struct {
+// actor is the per-combatant half of the simulation state — everything that
+// belongs to one player character rather than to the fight as a whole.
+//
+// It is embedded into combatState as a *pointer*, which promotes its fields:
+// `st.playerHP` still resolves, now to `st.actor.playerHP`. The embedded
+// pointer is a cursor naming whoever is currently resolving. Solo combat has a
+// one-element roster and never moves the cursor, so every read and every RNG
+// draw is bit-identical to the pre-roster engine — which is what keeps the
+// solo balance corpus (sim_results/d8prereq_corpus.jsonl lineage) valid.
+//
+// Field names are deliberately unchanged from when they lived on combatState.
+type actor struct {
+	// c is the Combatant this state belongs to. Nil only in tests that drive
+	// a primitive directly without a roster.
+	c *Combatant
+
 	playerHP int
-	enemyHP  int
 
 	// Consumable one-shots
 	healChargesLeft int // remaining heal-at-<50% triggers
@@ -304,13 +317,10 @@ type combatState struct {
 	reflectFrac     float64
 	autoCrit        bool
 
-	// Monster ability effects
+	// Monster ability effects landing on this actor
 	poisonTicks int
 	poisonDmg   int
 	stunPlayer  bool
-	enraged       bool
-	armorBroken   bool
-	armorBreakAmt float64
 
 	// Sovereign reprieve
 	deathSaveUsed bool
@@ -320,10 +330,6 @@ type combatState struct {
 	raged             bool // Orc Rage already triggered this fight
 	pendingRageAttack bool // next player attack gets +50% damage
 
-	// Phase 9 spell — enemy skip-first-attack (consumed on the first round
-	// the enemy would otherwise attack).
-	enemySkipFirst bool
-
 	// Phase 10 SUB2a-ii first-attack one-shots.
 	firstAttackBonusUsed  bool
 	assassinateRerollUsed bool
@@ -332,27 +338,52 @@ type combatState struct {
 	// Phase 10 SUB2b — Abjuration Arcane Ward HP buffer.
 	arcaneWardHP int
 
+	// Debuffs the enemy has stacked onto this actor specifically.
+	playerAtkDrain int // stat_drain: flat reduction to this actor's hit damage
+	playerACDebuff int // debuff: flat reduction to this actor's effective AC
+	maxHPDrain     int // max_hp_drain: reduction to this actor's effective MaxHP
+
 	// concentrationDmg — per-round damage of an active concentration AOE
-	// (Spirit Guardians et al.). Armed by a !cast of a concentration damage
-	// spell, ticked against the enemy every round_end until the fight ends
-	// or another concentration spell overwrites it. Only the turn engine
-	// reads it; SimulateCombat resolves whole fights in one pass and folds
-	// the aura's value into the picker's concentration multiplier instead.
+	// (Spirit Guardians et al.). Concentration is per-caster, so it lives
+	// here rather than on the fight.
 	concentrationDmg int
+}
+
+// combatState tracks mutable state during the simulation. The embedded *actor
+// is the cursor: the player character currently resolving. Everything declared
+// directly on combatState is shared by the whole fight — the enemy, the round
+// counter, the event log, and the RNG stream.
+type combatState struct {
+	*actor          // cursor into actors; promotes the per-actor fields
+	actors []*actor // the player roster, in seating order. len == 1 for solo.
+
+	enemyHP int
+
+	// Monster ability effects (enemy-side stance — shared across the roster)
+	enraged       bool
+	armorBroken   bool
+	armorBreakAmt float64
+
+	// Phase 9 spell — enemy skip-first-attack (consumed on the first round
+	// the enemy would otherwise attack). Holding the enemy holds it for
+	// everyone, so this is fight-scoped, not per-caster.
+	enemySkipFirst bool
 
 	// Phase 13 bestiary slice 3 — stateful monster-ability effects. Each is
 	// armed by applyAbility and read by the shared resolution primitives, so
 	// both engines honour them; the turn-based engine additionally round-trips
 	// them through CombatStatuses so they survive a suspend/resume.
+	//
+	// These describe the *enemy's* stance, so they are fight-scoped: an enemy
+	// holding a parry stance parries the next swing from anyone. The debuffs
+	// it stacks onto a specific character (stat_drain / debuff / max_hp_drain)
+	// live on actor instead.
 	enemyEvadeNext     bool    // evade: next player weapon attack auto-misses
 	enemyBlockUp       bool    // block: enemy holds a parry stance (~50% block on player hits)
 	enemyAdvantage     bool    // advantage: enemy rolls its attacks with advantage
 	enemyRetaliateFrac float64 // retaliate: fraction of a player hit reflected back
 	enemyRegen         int     // regenerate: enemy heals this much each round end
 	enemySurviveArmed  bool    // survive_at_1: enemy cheats death once, dropping to 1 HP
-	playerAtkDrain     int     // stat_drain: flat reduction to the player's hit damage
-	playerACDebuff     int     // debuff: flat reduction to the player's effective AC
-	maxHPDrain         int     // max_hp_drain: reduction to the player's effective MaxHP
 
 	// Phase 13 bestiary slice 4 — the former flavor-only placeholders, now
 	// backed by real state.
@@ -377,6 +408,47 @@ type combatState struct {
 // Auto-resolve leaves st.rng nil — behaviorally identical to the
 // pre-injection code; the turn-based engine and the timeout reaper seed
 // it per session so a fight can be resumed and replayed reproducibly.
+// newActor seats one player character, deriving its opening per-fight state
+// from that character's modifiers.
+func newActor(c *Combatant) *actor {
+	startHP := c.Stats.MaxHP
+	if c.Stats.StartHP > 0 && c.Stats.StartHP < c.Stats.MaxHP {
+		startHP = c.Stats.StartHP
+	}
+	a := &actor{
+		c:            c,
+		playerHP:     startHP,
+		wardCharges:  c.Mods.WardCharges,
+		sporeRounds:  c.Mods.SporeCloud,
+		reflectFrac:  c.Mods.ReflectNext,
+		autoCrit:     c.Mods.AutoCritFirst,
+		arcaneWardHP: c.Mods.ArcaneWardHP,
+	}
+	// HealItemCharges: explicit count overrides legacy one-shot. Backfill
+	// to 1 charge if the caller set a HealItem amount but no count.
+	a.healChargesLeft = c.Mods.HealItemCharges
+	if a.healChargesLeft == 0 && c.Mods.HealItem > 0 {
+		a.healChargesLeft = 1
+	}
+	return a
+}
+
+// seat points the cursor at roster index i. Every per-actor read in the
+// resolution primitives (st.playerHP, st.wardCharges, …) follows the cursor.
+// Solo combat seats index 0 once and never moves it.
+func (st *combatState) seat(i int) { st.actor = st.actors[i] }
+
+// anyAlive reports whether at least one seated character is still standing.
+// Solo fights read this as "the player is alive".
+func (st *combatState) anyAlive() bool {
+	for _, a := range st.actors {
+		if a.playerHP > 0 {
+			return true
+		}
+	}
+	return false
+}
+
 func (st *combatState) roll(n int) int     { return rngIntN(st.rng, n) }
 func (st *combatState) randFloat() float64 { return rngFloat(st.rng) }
 
@@ -398,23 +470,18 @@ func simulateCombatWithRNG(player, enemy Combatant, phases []CombatPhase, rng *r
 	if enemy.Stats.StartHP > 0 && enemy.Stats.StartHP < enemy.Stats.MaxHP {
 		enemyStart = enemy.Stats.StartHP
 	}
+	// Solo roster: one seat, cursor parked on it for the whole fight. The
+	// resolution primitives read the cursor, so this is the degenerate case
+	// of the N-player model and draws RNG in exactly the pre-roster order.
+	seat0 := newActor(&player)
+	seat0.playerHP = playerStart
 	st := &combatState{
-		playerHP:       playerStart,
+		actor:          seat0,
+		actors:         []*actor{seat0},
 		enemyHP:        enemyStart,
-		wardCharges:    player.Mods.WardCharges,
-		sporeRounds:    player.Mods.SporeCloud,
-		reflectFrac:    player.Mods.ReflectNext,
-		autoCrit:       player.Mods.AutoCritFirst,
 		enemySkipFirst: player.Mods.SpellEnemySkipFirst,
-		arcaneWardHP:   player.Mods.ArcaneWardHP,
 		rng:            rng,
 	}
-	// HealItemCharges: explicit count overrides legacy one-shot. Backfill
-	// to 1 charge if the caller set a HealItem amount but no count.
-	st.healChargesLeft = player.Mods.HealItemCharges
-	if st.healChargesLeft == 0 && player.Mods.HealItem > 0 {
-		st.healChargesLeft = 1
-	}
 
 	result := CombatResult{
 		PlayerStartHP: player.Stats.MaxHP,

internal/plugin/combat_roster_test.go

@@ -0,0 +1,153 @@
+package plugin
+
+// Roster/cursor tests for the N-player combat state (N3/P2).
+//
+// The solo path is pinned bit-for-bit by TestCombatCharacterization; these
+// cover the machinery that pin cannot see, because solo never moves the
+// cursor off seat 0.
+
+import (
+	"math/rand/v2"
+	"testing"
+)
+
+func TestNewActor_DerivesPerFightStateFromMods(t *testing.T) {
+	c := basePlayer()
+	c.Mods.WardCharges = 2
+	c.Mods.SporeCloud = 3
+	c.Mods.ReflectNext = 0.5
+	c.Mods.AutoCritFirst = true
+	c.Mods.ArcaneWardHP = 25
+
+	a := newActor(&c)
+
+	if a.c != &c {
+		t.Error("actor should point back at its Combatant")
+	}
+	if a.playerHP != c.Stats.MaxHP {
+		t.Errorf("playerHP = %d, want MaxHP %d", a.playerHP, c.Stats.MaxHP)
+	}
+	if a.wardCharges != 2 || a.sporeRounds != 3 || a.reflectFrac != 0.5 || !a.autoCrit || a.arcaneWardHP != 25 {
+		t.Errorf("consumable one-shots not carried from Mods: %+v", a)
+	}
+}
+
+func TestNewActor_StartHPAndHealChargeBackfill(t *testing.T) {
+	// StartHP below MaxHP means the character walks in wounded.
+	wounded := basePlayer()
+	wounded.Stats.StartHP = 40
+	if got := newActor(&wounded).playerHP; got != 40 {
+		t.Errorf("wounded entry HP = %d, want 40", got)
+	}
+
+	// StartHP at or above MaxHP is ignored (guards a stale snapshot).
+	full := basePlayer()
+	full.Stats.StartHP = 999
+	if got := newActor(&full).playerHP; got != full.Stats.MaxHP {
+		t.Errorf("StartHP above MaxHP should not raise entry HP, got %d", got)
+	}
+
+	// Legacy one-shot: a HealItem amount with no explicit count backfills to 1.
+	legacy := basePlayer()
+	legacy.Mods.HealItem = 30
+	if got := newActor(&legacy).healChargesLeft; got != 1 {
+		t.Errorf("legacy heal backfill = %d charges, want 1", got)
+	}
+
+	// An explicit count wins over the backfill.
+	stocked := basePlayer()
+	stocked.Mods.HealItem = 30
+	stocked.Mods.HealItemCharges = 4
+	if got := newActor(&stocked).healChargesLeft; got != 4 {
+		t.Errorf("explicit heal charges = %d, want 4", got)
+	}
+
+	// No HealItem at all means no charges, even if a count leaked through.
+	none := basePlayer()
+	if got := newActor(&none).healChargesLeft; got != 0 {
+		t.Errorf("no heal item should mean 0 charges, got %d", got)
+	}
+}
+
+// The cursor is the whole point of the embed: per-actor fields must follow
+// seat(), and fight-scoped fields must not.
+func TestCombatState_SeatSwitchesPerActorStateOnly(t *testing.T) {
+	alice, bob := basePlayer(), basePlayer()
+	alice.Name, bob.Name = "Alice", "Bob"
+	a0, a1 := newActor(&alice), newActor(&bob)
+
+	st := &combatState{
+		actor:   a0,
+		actors:  []*actor{a0, a1},
+		enemyHP: 100,
+		rng:     rand.New(rand.NewPCG(1, 1)),
+	}
+
+	// Wound seat 0 and burn one of its once-per-fight one-shots.
+	st.seat(0)
+	st.playerHP = 10
+	st.luckyUsed = true
+
+	// Seat 1 must be untouched — a promoted write goes to the cursor, not the
+	// struct. This is the assertion that would fail if actor were embedded by
+	// value instead of by pointer.
+	st.seat(1)
+	if st.playerHP != bob.Stats.MaxHP {
+		t.Errorf("seat 1 HP = %d, want a full pool %d — seat 0's wound leaked", st.playerHP, bob.Stats.MaxHP)
+	}
+	if st.luckyUsed {
+		t.Error("seat 1 saw seat 0's consumed Lucky reroll")
+	}
+	if st.c.Name != "Bob" {
+		t.Errorf("cursor Combatant = %q, want Bob", st.c.Name)
+	}
+
+	// Fight-scoped state is shared: writing it under one seat is visible
+	// under the other.
+	st.enemyHP = 42
+	st.enemyBlockUp = true
+	st.seat(0)
+	if st.playerHP != 10 || !st.luckyUsed {
+		t.Error("seat 0 lost its own state across a cursor round-trip")
+	}
+	if st.enemyHP != 42 || !st.enemyBlockUp {
+		t.Error("enemy stance should be fight-scoped, not per-actor")
+	}
+	if st.c.Name != "Alice" {
+		t.Errorf("cursor Combatant = %q, want Alice", st.c.Name)
+	}
+}
+
+func TestCombatState_AnyAlive(t *testing.T) {
+	alice, bob := basePlayer(), basePlayer()
+	a0, a1 := newActor(&alice), newActor(&bob)
+	st := &combatState{actor: a0, actors: []*actor{a0, a1}}
+
+	if !st.anyAlive() {
+		t.Error("a fresh roster should be alive")
+	}
+	a0.playerHP = 0
+	if !st.anyAlive() {
+		t.Error("one downed member should not end the fight while another stands")
+	}
+	a1.playerHP = 0
+	if st.anyAlive() {
+		t.Error("a fully downed roster should not be alive")
+	}
+}
+
+// Solo fights seat exactly one actor and park the cursor on it. If this ever
+// regresses, the characterization golden stops proving anything about the
+// production auto-resolve path.
+func TestSimulateCombat_SeatsExactlyOneActor(t *testing.T) {
+	p, e := basePlayer(), baseEnemy()
+	seat0 := newActor(&p)
+	st := &combatState{actor: seat0, actors: []*actor{seat0}, enemyHP: e.Stats.MaxHP}
+
+	if len(st.actors) != 1 {
+		t.Fatalf("solo roster length = %d, want 1", len(st.actors))
+	}
+	if st.actor != st.actors[0] {
+		t.Error("solo cursor must point at seat 0")
+	}
+}

internal/plugin/combat_turn_engine.go

@@ -111,17 +111,15 @@ func phaseOrdinal(phase string) uint64 {
 // resumeTurnEngine rebuilds the in-memory combatState from a persisted session.
 // rng is the deterministic source for this step (see combatSessionRNG).
 func resumeTurnEngine(sess *CombatSession, player, enemy *Combatant, rng *rand.Rand) *turnEngine {
-	st := &combatState{
+	// The seated character's half of the persisted statuses. A single-seat
+	// roster today; P4 gives combat_session per-participant rows and this
+	// becomes one actor per member.
+	seat0 := &actor{
+		c:           player,
 		playerHP:    sess.PlayerHP,
-		enemyHP:        sess.EnemyHP,
-		round:          sess.Round,
 		poisonTicks: sess.Statuses.PoisonTicks,
 		poisonDmg:   sess.Statuses.PoisonDmg,
 		stunPlayer:  sess.Statuses.StunPlayer,
-		enraged:        sess.Statuses.Enraged,
-		armorBroken:    sess.Statuses.ArmorBroken,
-		armorBreakAmt:  sess.Statuses.ArmorBreakAmt,
-		enemySkipFirst: sess.Statuses.EnemySkipNext,
 		// Fight-scoped depleting resources + once-per-fight one-shots: restored
 		// from the persisted statuses so a charge or "already used" flag can't
 		// reset across a suspend/resume. commit writes the updated values back.
@@ -139,6 +137,20 @@ func resumeTurnEngine(sess *CombatSession, player, enemy *Combatant, rng *rand.R
 		firstAttackBonusUsed:  sess.Statuses.FirstAtkBonusUsed,
 		assassinateRerollUsed: sess.Statuses.AssassinateReroll,
 		assassinateBonusUsed:  sess.Statuses.AssassinateBonus,
+		// Enemy debuffs stacked onto this character specifically.
+		playerAtkDrain: sess.Statuses.PlayerAtkDrain,
+		playerACDebuff: sess.Statuses.PlayerACDebuff,
+		maxHPDrain:     sess.Statuses.MaxHPDrain,
+	}
+	st := &combatState{
+		actor:          seat0,
+		actors:         []*actor{seat0},
+		enemyHP:        sess.EnemyHP,
+		round:          sess.Round,
+		enraged:        sess.Statuses.Enraged,
+		armorBroken:    sess.Statuses.ArmorBroken,
+		armorBreakAmt:  sess.Statuses.ArmorBreakAmt,
+		enemySkipFirst: sess.Statuses.EnemySkipNext,
 		// Slice-3 stateful monster-ability effects — armed by applyAbility,
 		// round-tripped here so they survive a suspend/resume or reaper auto-play.
 		enemyEvadeNext:     sess.Statuses.EnemyEvadeNext,
@@ -147,9 +159,6 @@ func resumeTurnEngine(sess *CombatSession, player, enemy *Combatant, rng *rand.R
 		enemyRetaliateFrac: sess.Statuses.EnemyRetaliateFrac,
 		enemyRegen:         sess.Statuses.EnemyRegen,
 		enemySurviveArmed:  sess.Statuses.EnemySurviveArmed,
-		playerAtkDrain:     sess.Statuses.PlayerAtkDrain,
-		playerACDebuff:     sess.Statuses.PlayerACDebuff,
-		maxHPDrain:         sess.Statuses.MaxHPDrain,
 		// Slice-4 monster-ability effects — the former flavor-only placeholders.
 		enemySpellResist: sess.Statuses.EnemySpellResist,
 		enemyRevealNext:  sess.Statuses.EnemyRevealNext,