N3/P3: initiative, and a turn engine that seats a party

The turn engine ran a fixed player -> enemy -> round_end phase machine over
one player and one monster. It now runs a round as a sequence of seats.

turnOrder derives that sequence per round. A solo roster short-circuits to
the historical [player, enemy] and rolls nothing -- the duel has never had
initiative, and handing the monster a coin flip on who swings first would be
a live balance change. A party rolls it with the auto-resolve engine's own
formula (speed + d10 + InitiativeBias).

Every seat in a round shares a (round, phase) pair, so the acting seat is
mixed into the RNG *seed* rather than the stream. Seat 0 and the enemy
sentinel mix to nothing, which is what keeps a solo fight drawing exactly
the pre-roster stream across a suspend/resume.

The round cursor persists as Statuses.TurnIdx, omitempty so no solo row
carries it. A fight that was in flight when the field landed decodes it as
0; turnIdxForPhase reconciles that against Phase, which is the older and
load-bearing field. Without it, a suspended enemy_turn would resume, step,
and land back on enemy_turn forever.

The enemy now picks a target uniformly among the standing roster (solo draws
nothing), a downed seat forfeits its turn silently, and the fight is lost
only when anyAlive() goes false -- not when the acting seat drops. That last
one fixes a latent solo bug on the way past: resolvePlayerSwings returns
false when a retaliate aura kills the swinger between extra attacks, and the
old code walked that corpse into the enemy's turn.

commit() reads seat 0 explicitly instead of the cursor, which the enemy turn
parks on its target and round_end walks across the roster.

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

internal/plugin/combat_engine.go

@@ -309,6 +309,11 @@ type actor struct {
 	c *Combatant
 
 	playerHP int
+	// hpMax is this actor's HP ceiling for in-fight healing. It tracks
+	// c.Stats.MaxHP except in the turn-based engine, where seat 0 restores the
+	// session's persisted player_hp_max — that snapshot comes from
+	// dndHPSnapshot and can differ from the rebuilt combatant's MaxHP.
+	hpMax int
 
 	// Consumable one-shots
 	healChargesLeft int // remaining heal-at-<50% triggers
@@ -418,6 +423,7 @@ func newActor(c *Combatant) *actor {
 	a := &actor{
 		c:            c,
 		playerHP:     startHP,
+		hpMax:        c.Stats.MaxHP,
 		wardCharges:  c.Mods.WardCharges,
 		sporeRounds:  c.Mods.SporeCloud,
 		reflectFrac:  c.Mods.ReflectNext,