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_engine.go

@@ -211,6 +211,15 @@ type CombatEvent struct {
 	// Roll is the raw d20 (1..20); RollAgainst is the target AC.
 	Roll        int
 	RollAgainst int
+	// Seat is the roster index of the character this event is about: the one
+	// who swung, whose pet struck, who the enemy hit, whose poison ticked. The
+	// turn engine stamps it (turnEngine.stampSeat); the auto-resolve engine
+	// leaves it 0, which is correct for its one and only combatant.
+	//
+	// It exists so a party's play-by-play can name the right person. Solo events
+	// are all seat 0, and the omitempty tag keeps the field out of every solo
+	// turn_log_json — rows written before N3/P5 decode unchanged.
+	Seat int `json:"Seat,omitempty"`
 }
 
 type CombatResult struct {
@@ -361,6 +370,10 @@ type actor struct {
 type combatState struct {
 	*actor          // cursor into actors; promotes the per-actor fields
 	actors []*actor // the player roster, in seating order. len == 1 for solo.
+	// seatIdx is the roster index the cursor points at. Kept in step with the
+	// embedded *actor by seat(); read only by the turn engine, to attribute the
+	// events a phase emitted to the character they happened to.
+	seatIdx int
 
 	enemyHP int
 
@@ -442,7 +455,9 @@ func newActor(c *Combatant) *actor {
 // 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] }
+// seat moves the cursor to a roster index. seatIdx trails it so the turn engine
+// can tag the events a phase emits with the character they are about.
+func (st *combatState) seat(i int) { st.actor, st.seatIdx = st.actors[i], i }
 
 // anyAlive reports whether at least one seated character is still standing.
 // Solo fights read this as "the player is alive".