N3/P4: give every seat a row of its own

The turn engine seats a party since P3, but only seat 0 survived a
suspend: seats 1+ reopened from their Mods on every step, rearming their
once-per-fight one-shots -- a party Halfling rerolled a nat 1 every round.

Split CombatStatuses into the fight-scoped half (the enemy's stance, the
round cursor) and the per-character half, ActorStatuses. The embed is
anonymous and untagged, so statuses_json stays the same flat object it
always was and every in-flight prod row decodes unchanged.

Seat 0 keeps living on combat_session. Seats 1+ get combat_participant
rows. That asymmetry is the point: a solo fight -- which is every fight
that has ever run, and the whole balance corpus -- writes exactly the
bytes it wrote before, and no participant rows at all. roster_size
guards the read, so the solo loader never issues the second query.

Parties commit their seats in the same transaction as the session row;
solo keeps its single unwrapped UPDATE.

expedition_party is the co-op roster. No party_id on dnd_expedition:
expedition_id already identifies the party, and a second key would be a
second answer to "who is in this party". Absent means solo, in both new
tables, so neither needs a bootstrap backfill.

The combat characterization golden is byte-identical.

Also seeds and re-powers the two statistical subclass tests. They drew
from the package-global RNG, so their verdict depended on how much
randomness every test declared before them happened to consume -- which
is why they flaked on a clean tree. Sweeping 40 seeds: Precision Attack
had a mean margin of +127 wins against a +50 threshold but a worst case
of -42, and Assassinate averaged +12.8 with two seeds outright negative.
Both effects are real; the trial counts were too low to see them. Seeded,
and raised to 24000/6000 trials, where all 40 seeds clear.

internal/plugin/combat_session_test.go

@@ -52,7 +52,7 @@ func TestStartCombatSession_RoundTrip(t *testing.T) {
 	got.Phase = CombatPhaseRoundEnd
 	got.PlayerHP = 40
 	got.EnemyHP = 15
-	got.Statuses = CombatStatuses{PoisonTicks: 2, PoisonDmg: 5, Enraged: true}
+	got.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 2, PoisonDmg: 5}, Enraged: true}
 	got.TurnLog = append(got.TurnLog, CombatEvent{Round: 3, Actor: "player", Action: "hit", Damage: 9})
 	if err := saveCombatSession(got); err != nil {
 		t.Fatalf("saveCombatSession: %v", err)
@@ -67,7 +67,7 @@ func TestStartCombatSession_RoundTrip(t *testing.T) {
 	if reloaded.PlayerHP != 40 || reloaded.EnemyHP != 15 {
 		t.Errorf("hp not persisted: %+v", reloaded)
 	}
-	if reloaded.Statuses != (CombatStatuses{PoisonTicks: 2, PoisonDmg: 5, Enraged: true}) {
+	if reloaded.Statuses != (CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 2, PoisonDmg: 5}, Enraged: true}) {
 		t.Errorf("statuses not persisted: %+v", reloaded.Statuses)
 	}
 	if len(reloaded.TurnLog) != 1 || reloaded.TurnLog[0].Action != "hit" {
@@ -223,7 +223,7 @@ func TestTurnEngine_PhaseProgression(t *testing.T) {
 
 func TestTurnEngine_PoisonTickAtRoundEnd(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 80)
-	sess.Statuses = CombatStatuses{PoisonTicks: 2, PoisonDmg: 7}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 2, PoisonDmg: 7}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	events, err := stepEngine(sess, &player, &enemy, PlayerAction{})
@@ -246,7 +246,7 @@ func TestTurnEngine_PoisonTickAtRoundEnd(t *testing.T) {
 
 func TestTurnEngine_PoisonTickCanBeLethal(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 4, 80)
-	sess.Statuses = CombatStatuses{PoisonTicks: 1, PoisonDmg: 9}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 1, PoisonDmg: 9}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	if _, err := stepEngine(sess, &player, &enemy, PlayerAction{}); err != nil {
@@ -265,7 +265,7 @@ func TestTurnEngine_PoisonTickCanBeLethal(t *testing.T) {
 
 func TestTurnEngine_ConcentrationTickAtRoundEnd(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 80)
-	sess.Statuses = CombatStatuses{ConcentrationDmg: 12}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{ConcentrationDmg: 12}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	events, err := stepEngine(sess, &player, &enemy, PlayerAction{})
@@ -288,7 +288,7 @@ func TestTurnEngine_ConcentrationTickAtRoundEnd(t *testing.T) {
 
 func TestTurnEngine_ConcentrationTickCanBeLethal(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 9)
-	sess.Statuses = CombatStatuses{ConcentrationDmg: 12}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{ConcentrationDmg: 12}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	if _, err := stepEngine(sess, &player, &enemy, PlayerAction{}); err != nil {
@@ -433,7 +433,7 @@ func TestApplyBuffDelta(t *testing.T) {
 
 func TestApplySessionBuffs(t *testing.T) {
 	player := basePlayer() // AC 0, AttackBonus 0, CritRate 0.05, Speed 10, DamageReduct 1.0
-	applySessionBuffs(&player, CombatStatuses{
+	applySessionBuffs(&player, ActorStatuses{
 		BuffACBonus: 3, BuffAtkBonus: 2, BuffSpeedBonus: 5, BuffCritRate: 0.15,
 		BuffDamageBonus: 0.25, BuffDamageReductMul: 0.8, BuffPetProc: 0.5, BuffPetDmg: 6,
 	})
@@ -473,7 +473,7 @@ func TestSeedCombatSessionOneShots(t *testing.T) {
 // re-bank a depleted ward charge — on every resumed round.
 func TestTurnEngine_OneShotsRoundTrip(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 50)
-	sess.Statuses = CombatStatuses{
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{
 		WardCharges: 3, SporeRounds: 2, ReflectFrac: 0.5, AutoCritFirst: true,
 		ArcaneWardHP: 10, HealChargesLeft: 1,
 		Raged: true, LuckyUsed: true, DeathSaveUsed: true, PendingRage: true,
@@ -481,7 +481,7 @@ func TestTurnEngine_OneShotsRoundTrip(t *testing.T) {
 		// Buff stat deltas are owned by the command layer — commit must leave
 		// them untouched rather than zeroing them on every step.
 		BuffACBonus: 2, BuffDamageBonus: 0.15,
-	}
+	}}
 	want := sess.Statuses // round_end with no poison mutates none of these
 	player, enemy := basePlayer(), baseEnemy()