N3/P5: a fight that knows whose turn it is

A solo fight is a conversation: the player types, the engine answers, and
nothing happens in between. A party fight is a queue, and three things follow.

Turn ownership. Only the seat on the clock may act, so beginCombatTurn resolves
the sender's seat and refuses the rest before anyone spends a slot or burns an
item. A fight lock, because three members typing !attack at once took three
different user locks and the check would have passed for all three: it takes the
fight's lock (keyed on seat 0) then the member's own, always in that order, and
a solo fight -- whose owner is the sender, and sync.Mutex is not reentrant --
takes exactly the one lock it always took. And a turn deadline, because one
member who wanders off must not freeze the other two for the hour it takes the
session reaper to wake up.

The deadline is three minutes, not the plan's sixty seconds. The sweep rides the
existing one-minute ticker, so any deadline really fires in [d, d+1m); and
expeditions here run for days, so the asymmetry favours patience over robbing
someone of their boss turn while they read the room on their phone. A lapse
latches that seat onto the auto-picker for the rest of the fight, so an absent
member costs the party one wait rather than one per round. Typing anything hands
the wheel back. Solo is never swept.

Three seat-0 leaks fixed on the way past, all of which would have surfaced as
the leader quietly doing everyone's business:

  - mid-fight buffs folded into the session's embedded ActorStatuses, so a
    member casting Shield on themselves would have armoured the leader;
  - pickAutoCombatAction read sess.PlayerHP and Statuses.ConcentrationDmg, so
    playing an away member's turn would have healed the wrong person and
    re-armed the wrong aura;
  - runCombatRound rested on any player_turn, and a downed seat still holds one
    -- the round would have come to rest on a corpse and waited for a dead
    member to type !attack. settleCombatSession drains it. beginCombatTurn
    settles before reading the clock, which also fixes a latent solo bug: a
    fight interrupted mid enemy_turn resumed parked there and silently ate the
    player's next !attack.

The narration turned out to be written in the second person -- "You score 9
damage", "A hit gets through your guard" -- so swapping a name per seat would
have told three people they each landed the same blow. A round is rendered once
per reader instead: your own events go through the untouched flavor pool, your
allies' through a terse third-person summary. CombatEvent carries the seat to
make that possible, stamped once per phase step rather than at the twenty-odd
append sites in the primitives, which emit against the cursor and know nothing
of seats.

Closing out fans along the seam the data model already cut. Threat, the
zone-kill record, the boss-defeat drop and the run teardown all resolve through
getActiveExpedition or getActiveZoneRun, and a member owns neither row -- so
they fire once, for the owner. Fanning them out would have tripled the threat a
single kill costs. HP, XP, loot and death are the character's, and every seat
gets their own. A member can be dead in a fight the party won, so death is read
per seat off HP, not off the session's status.

The reaper stays attack-only. Finishing an abandoned fight should not quietly
burn the player's spell slots and potions; the deadline latch does use the
picker, because that member is mid-fight with a party waiting on them.

startPartyCombatSession has no production caller yet -- handleFightCmd still
opens a solo session. P6 seats the party.

TestCombatCharacterization is byte-identical: solo balance did not move.

internal/plugin/combat_turn_engine.go

@@ -195,6 +195,31 @@ func phaseForSeat(seat int) string {
 	return CombatPhasePlayerTurn
 }
 
+// stepSeat is the seat that resolves the session's current phase: the acting
+// player on a player_turn, the enemy sentinel on anything else.
+//
+// It is the *single* derivation of "whose step is this". advancePartyCombatSession
+// needs it before the engine is resumed, because the seat seeds that step's RNG;
+// the command layer needs it to tell a member it is not their turn. Both read
+// only (sessionID, round, roster), so they agree by construction.
+func stepSeat(sess *CombatSession, players []*Combatant, enemy *Combatant) int {
+	if sess.Phase != CombatPhasePlayerTurn {
+		return enemySeat
+	}
+	order := turnOrder(sess, sess.Round, players, enemy)
+	return order[turnIdxForPhase(order, sess.Statuses.TurnIdx, sess.Phase)]
+}
+
+// actingSeat names the player seat currently on the clock. The false return
+// means the fight is not waiting on anybody: it is mid enemy-turn, mid
+// round-end, or over.
+func actingSeat(sess *CombatSession, players []*Combatant, enemy *Combatant) (int, bool) {
+	if !sess.IsActive() || sess.Phase != CombatPhasePlayerTurn {
+		return 0, false
+	}
+	return stepSeat(sess, players, enemy), true
+}
+
 // turnIdxForPhase reconciles a persisted cursor against the persisted phase.
 // They can disagree for exactly one reason: a fight that was in flight when
 // TurnIdx was introduced decodes as 0 regardless of whose turn it is. Phase is
@@ -362,6 +387,18 @@ func (te *turnEngine) advance() {
 // events generated this step are returned (also accumulated by commit into
 // sess.TurnLog). It does not persist — call commit then saveCombatSession, or
 // use advanceCombatSession which does both.
+// stampSeat tags every event appended since mark with the roster seat it is
+// about, so a party's play-by-play can name the right character. The primitives
+// in combat_primitives.go emit against the cursor and know nothing of seats, so
+// the tag is applied here, once per phase step, rather than at ~20 append sites.
+//
+// Solo stamps seat 0 onto seat-0 events — a no-op that omitempty erases.
+func (te *turnEngine) stampSeat(mark, seat int) {
+	for i := mark; i < len(te.st.events); i++ {
+		te.st.events[i].Seat = seat
+	}
+}
+
 func (te *turnEngine) step(action PlayerAction) ([]CombatEvent, error) {
 	if !te.sess.IsActive() {
 		return nil, errCombatSessionOver
@@ -369,9 +406,18 @@ func (te *turnEngine) step(action PlayerAction) ([]CombatEvent, error) {
 	te.st.events = nil
 	switch te.sess.Phase {
 	case CombatPhasePlayerTurn:
+		// The cursor was seated on the acting player by resumeTurnEngine, and
+		// stepPlayerTurn never moves it, so every event this phase emits — the
+		// swings, the pet, the spirit weapon, the retaliate that kills them —
+		// belongs to that seat.
+		acting := te.st.seatIdx
 		te.stepPlayerTurn(action)
+		te.stampSeat(0, acting)
 	case CombatPhaseEnemyTurn:
 		te.stepEnemyTurn()
+		// stepEnemyTurn seats its target before anything resolves, and the whole
+		// phase lands on that one character.
+		te.stampSeat(0, te.st.seatIdx)
 	case CombatPhaseRoundEnd:
 		te.stepRoundEnd()
 	case CombatPhaseOver:
@@ -707,6 +753,7 @@ func (te *turnEngine) stepRoundEnd() {
 		if st.poisonTicks <= 0 {
 			continue
 		}
+		mark := len(st.events)
 		st.playerHP = max(0, st.playerHP-st.poisonDmg)
 		st.poisonTicks--
 		st.events = append(st.events, CombatEvent{
@@ -714,9 +761,11 @@ func (te *turnEngine) stepRoundEnd() {
 			Damage: st.poisonDmg, PlayerHP: st.playerHP, EnemyHP: st.enemyHP,
 		})
 		if st.playerHP <= 0 && !trySave(st, st.c, CombatPhaseRoundEnd) && !st.anyAlive() {
+			te.stampSeat(mark, i)
 			te.finish(CombatStatusLost)
 			return
 		}
+		te.stampSeat(mark, i)
 	}
 	// Concentration aura (Spirit Guardians et al.): the lingering spell bites
 	// the enemy each round it stays up. Concentration is per-caster, so every
@@ -730,7 +779,7 @@ func (te *turnEngine) stepRoundEnd() {
 		st.enemyHP = max(0, st.enemyHP-st.concentrationDmg)
 		st.events = append(st.events, CombatEvent{
 			Round: st.round, Phase: CombatPhaseRoundEnd, Actor: "player", Action: "concentration_tick",
-			Damage: st.concentrationDmg, PlayerHP: st.playerHP, EnemyHP: st.enemyHP,
+			Damage: st.concentrationDmg, PlayerHP: st.playerHP, EnemyHP: st.enemyHP, Seat: i,
 		})
 		if st.enemyHP <= 0 {
 			te.finish(CombatStatusWon)
@@ -830,12 +879,7 @@ func advancePartyCombatSession(sess *CombatSession, players []*Combatant, enemy
 	// step's RNG — so the order is derived once here and once inside
 	// resumeTurnEngine. Both derivations read only (sessionID, round, roster),
 	// so they agree by construction.
-	seat := enemySeat
-	if sess.Phase == CombatPhasePlayerTurn {
-		order := turnOrder(sess, sess.Round, players, enemy)
-		seat = order[turnIdxForPhase(order, sess.Statuses.TurnIdx, sess.Phase)]
-	}
-	rng := combatSessionStepRNG(sess, seat)
+	rng := combatSessionStepRNG(sess, stepSeat(sess, players, enemy))
 	te := resumeTurnEngine(sess, players, enemy, rng)
 	events, err := te.step(action)
 	if err != nil {