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_party_finish.go

@@ -0,0 +1,183 @@
+package plugin
+
+import (
+	"fmt"
+	"log/slog"
+	"strings"
+
+	"maunium.net/go/mautrix/id"
+)
+
+// N3/P5 — closing out a party fight.
+//
+// finishCombatSession (combat_cmd.go) is the solo close-out and stays exactly
+// what it was. This is its sibling for a seated roster, and the split it makes
+// is the one the data model already forced:
+//
+//   - Run- and expedition-scoped effects fire ONCE, for the owner. Threat, the
+//     zone-kill record, the boss-defeat threat drop, the run teardown on a loss
+//     — every one of them resolves through getActiveExpedition(userID) or
+//     getActiveZoneRun(userID), and a party member owns neither row. Fanning
+//     them out would be a no-op for members and would triple the threat the
+//     owner pays for a single kill.
+//   - Character-scoped effects fan out. HP, XP, loot, and death belong to the
+//     person, not the expedition, and every seat gets their own.
+//
+// A member can be dead in a fight the party won: they dropped, the others
+// finished the job. So death is decided per seat off HP, not off the session's
+// terminal status.
+
+// finishPartyCombatSession runs the terminal side effects for a seated roster
+// and returns each seat's own player-facing close-out, indexed by seat. A solo
+// session delegates to finishCombatSession untouched.
+func (p *AdventurePlugin) finishPartyCombatSession(ct *combatTurn) []string {
+	sess, enemy := ct.sess, *ct.enemy
+	owner := id.UserID(sess.UserID)
+	if !ct.isParty() {
+		return []string{p.finishCombatSession(owner, sess, enemy)}
+	}
+
+	run, _ := getZoneRun(sess.RunID)
+	var zone ZoneDefinition
+	elite := true
+	cadence := sess.Round
+	if run != nil {
+		zone = zoneOrFallback(run.ZoneID)
+		elite = run.CurrentRoomType() != RoomBoss
+		cadence = narrationCadence(run)
+	}
+	monster := dndBestiary[sess.EnemyID]
+
+	// nat20/nat1 mood deltas from the whole fight's event log — the run's, so once.
+	scanMoodEventsFromEvents(sess.RunID, sess.TurnLog)
+
+	// Every seat's HP lands on their sheet regardless of how the fight ended.
+	for seat := range sess.RosterSize() {
+		persistDnDHPAfterCombat(id.UserID(sess.seatUserID(seat)), sess.seatHP(seat))
+	}
+
+	switch sess.Status {
+	case CombatStatusWon:
+		return p.finishPartyWin(ct, run, zone, monster, elite, cadence)
+	case CombatStatusLost:
+		return p.finishPartyLoss(ct, zone, cadence)
+	case CombatStatusFled:
+		_ = abandonZoneRun(owner)
+		forceExtractExpeditionForRunLoss(owner, "combat flee")
+		return p.eachSeat(ct, fmt.Sprintf(
+			"🏃 The party broke off from **%s** and slipped away. Run ended — you keep your wounds, but you live.", enemy.Name))
+	default:
+		return p.eachSeat(ct, "The fight is over.")
+	}
+}
+
+// finishPartyWin pays the party out. The kill is the expedition's, so threat and
+// the zone-kill record resolve once through the owner; XP and loot are the
+// character's, so every member rolls their own — loot independently, per the C1
+// no-split rule, and XP in full, per accessibility over crunch.
+func (p *AdventurePlugin) finishPartyWin(
+	ct *combatTurn, run *DungeonRun, zone ZoneDefinition, monster DnDMonsterTemplate, elite bool, cadence int,
+) []string {
+	sess := ct.sess
+	owner := id.UserID(sess.UserID)
+
+	recordZoneKillForUser(owner, sess.EnemyID)
+	applyRoomCombatThreatForUser(owner, elite)
+
+	bossOnExpedition := false
+	if !elite {
+		if exp, eerr := getActiveExpedition(owner); eerr == nil && exp != nil {
+			_ = applyBossDefeatThreat(exp.ID)
+			bossOnExpedition = true
+		}
+	}
+
+	tier := 1
+	if run != nil {
+		tier = int(zone.Tier)
+	}
+	shared := twinBeeLine(zone.ID, DMCombatEnd, sess.RunID, cadence)
+	emoji := "✅"
+	if !elite {
+		emoji = "🏆"
+	}
+
+	out := make([]string, sess.RosterSize())
+	for seat := range sess.RosterSize() {
+		uid := id.UserID(sess.seatUserID(seat))
+		hp, hpMax := sess.seatHP(seat), sess.seatHPMax(seat)
+
+		// A member who went down before the killing blow still earns the kill —
+		// but at a fifth of their pool or less, the XP path calls it near-death.
+		nearDeath := hpMax > 0 && hp*5 < hpMax
+		if xp := zoneCombatXP(CombatResult{PlayerWon: true, NearDeath: nearDeath}, monster.CR, tier); xp > 0 {
+			if _, err := p.grantDnDXP(uid, xp); err != nil {
+				slog.Error("combat: party grantDnDXP", "user", uid, "err", err)
+			}
+		}
+
+		var b strings.Builder
+		if shared != "" && seat == 0 {
+			b.WriteString(shared + "\n")
+		}
+		b.WriteString(fmt.Sprintf("%s **%s** down. The party stands.\n", emoji, ct.enemy.Name))
+		if hp <= 0 {
+			// They won it from the floor. Down is down: the hospital takes them.
+			markAdventureDead(uid, "zone", zone.Display)
+			b.WriteString("💀 You didn't see it fall — you were already down.\n")
+		} else {
+			b.WriteString(fmt.Sprintf("You finished at **%d/%d HP**.\n", hp, hpMax))
+			if drop := p.dropZoneLoot(uid, zone.ID, monster, !elite, elite); drop != "" {
+				b.WriteString(drop + "\n")
+			}
+		}
+		switch {
+		case bossOnExpedition && seat == 0:
+			b.WriteString("🎉 **Zone cleared — the expedition is won.** `!expedition run` to march out and claim your spoils.")
+		case bossOnExpedition:
+			b.WriteString("🎉 **Zone cleared — the expedition is won.** Your leader marches the party out.")
+		case seat == 0:
+			b.WriteString(continueHint(owner))
+		default:
+			b.WriteString("Waiting on your leader to move the party on.")
+		}
+		out[seat] = b.String()
+	}
+	return out
+}
+
+// finishPartyLoss ends the run for everyone. The whole roster is down — the turn
+// engine only reports Lost once anyAlive() goes false — so every member takes
+// the death, and the owner's run and expedition are torn down once.
+func (p *AdventurePlugin) finishPartyLoss(ct *combatTurn, zone ZoneDefinition, cadence int) []string {
+	sess := ct.sess
+	owner := id.UserID(sess.UserID)
+
+	if run, _ := getZoneRun(sess.RunID); run != nil {
+		_, _ = applyMoodEvent(sess.RunID, MoodEventPlayerDeath)
+	}
+	_ = abandonZoneRun(owner)
+	forceExtractExpeditionForRunLoss(owner, "combat death")
+
+	line := twinBeeLine(zone.ID, DMPlayerDeath, sess.RunID, cadence)
+	out := make([]string, sess.RosterSize())
+	for seat := range sess.RosterSize() {
+		markAdventureDead(id.UserID(sess.seatUserID(seat)), "zone", zone.Display)
+		var b strings.Builder
+		if line != "" && seat == 0 {
+			b.WriteString(line + "\n")
+		}
+		b.WriteString(fmt.Sprintf("💀 The party fell to **%s**. Run ended.", ct.enemy.Name))
+		out[seat] = b.String()
+	}
+	return out
+}
+
+// eachSeat gives every seat the same close-out block.
+func (p *AdventurePlugin) eachSeat(ct *combatTurn, block string) []string {
+	out := make([]string, ct.sess.RosterSize())
+	for i := range out {
+		out[i] = block
+	}
+	return out
+}