Adv 2.0 L2: rip legacy arena combat path; boss flow is the only path

Cancels the planned ARENA_BOSS_FLOW soak. The boss flow is shipped
unconditionally with no legacy fallback — boss-flow errors now surface
to the player and abort the round rather than silently falling back to
the old CombatPower path.

Deleted:
- runArenaCombat (combat_bridge.go) — legacy CombatPower-vs-Lethality
  arena combat driver.
- RenderCombatLogArena, renderArenaOutcome (combat_narrative.go) — legacy
  arena renderers. RenderCombatLogArena was a thin alias for
  RenderCombatLog; renderArenaOutcome was already dead.
- renderArenaCombatFinalMessage (combat_bridge.go) — legacy "rewards +
  closer" trailing text.
- arenaWinCloser, arenaLoseCloser (adventure_arena_combat.go) — flavor
  helpers that only fed the deleted renderers. File now just holds
  arenaParticipationXP.
- arenaBossFlowEnabled + the ARENA_BOSS_FLOW env var (adventure_arena.go,
  .env.example). The env gate served only the now-removed fallback.
- sendArenaCombatMessages (adventure_arena.go) — wrapper that switched
  pacing between boss flow and legacy. Replaced with direct
  sendZoneCombatMessages calls at the three call sites.
- TestArenaBossFlowEnabled (bossflow test) and
  TestRenderCombatLogArena_ProducesPhaseMessages (narrative test).

Simplified resolveArenaRound / resolveArenaSurvival / resolveArenaDeath:
the bossNarr-vs-nil branches collapsed since narration is now always
produced by resolveArenaBoss. Equipment degradation and the death-save
reprieve hook are unchanged — both already worked off the boss-flow
CombatResult.

Migration doc (§4) updated to record the cancellation and the
remaining tuning approach (playtest-driven, no flag soak window).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/combat_bridge.go

@@ -10,99 +10,6 @@ import (
 	"maunium.net/go/mautrix/id"
 )
 
-// runArenaCombat executes arena combat using the new simulation engine.
-// Returns the CombatResult and the post-combat Misty condition repair amount (if any).
-func (p *AdventurePlugin) runArenaCombat(
-	userID id.UserID,
-	char *AdventureCharacter,
-	equip map[EquipmentSlot]*AdvEquipment,
-	monster *ArenaMonster,
-	arenaRound int,
-	arenaTier int,
-) (CombatResult, int) {
-	bonuses := p.loadCombatBonuses(userID, char)
-	chatLvl := p.chatLevel(userID)
-	hasGrudge := char.GrudgeLocation != ""
-
-	playerStats, playerMods := DerivePlayerStats(char, equip, bonuses, chatLvl, char.CurrentStreak, hasGrudge)
-
-	// Load consumables from inventory and auto-select
-	consumables := p.loadConsumableInventory(userID)
-	enemyStats, _ := DeriveArenaMonsterStats(monster)
-
-	// All combat is D&D. If the player has no sheet yet (or only a draft),
-	// auto-migrate them to a sensible inferred character before fighting.
-	dndChar, freshMigrate, err := ensureDnDCharacterForCombat(userID, char)
-	if err != nil {
-		slog.Error("dnd: ensureDnDCharacterForCombat (arena) failed", "user", userID, "err", err)
-		return CombatResult{}, 0
-	}
-	if freshMigrate {
-		p.maybeSendDnDOnboarding(userID, char, dndChar)
-	}
-	applyDnDPlayerLayer(&playerStats, dndChar)
-	applyDnDEquipmentLayer(&playerStats, dndChar, equip)
-	applyDnDHPScaling(&playerStats, dndChar)
-	applyClassPassives(&playerStats, &playerMods, dndChar)
-	applyRacePassives(&playerStats, &playerMods, dndChar)
-	applySubclassPassives(&playerStats, &playerMods, dndChar)
-	if firedName, fired := applyArmedAbility(dndChar, &playerMods); fired {
-		slog.Info("dnd: armed ability fired", "user", userID, "ability", firedName)
-	}
-	applyDnDArenaMonsterLayer(&enemyStats, monster.ThreatLevel)
-	applyPendingCast(userID, dndChar, &playerStats, &playerMods, &enemyStats)
-
-	selected := SelectConsumables(consumables, playerStats, enemyStats, arenaRound, arenaTier)
-	ApplyConsumableMods(&playerStats, &playerMods, selected)
-
-	// Misty condition repair (post-combat, not part of engine)
-	condRepair := 0
-	now := time.Now().UTC()
-	if char.MistyBuffExpires != nil && now.Before(*char.MistyBuffExpires) {
-		if rand.Float64() < 0.20 {
-			condRepair = 5
-		}
-	}
-
-	player := Combatant{
-		Name:     char.DisplayName,
-		Stats:    playerStats,
-		Mods:     playerMods,
-		IsPlayer: true,
-	}
-	enemy := Combatant{
-		Name:    monster.Name,
-		Stats:   enemyStats,
-		Mods:    CombatModifiers{DamageReduct: 1.0},
-		Ability: monster.Ability,
-	}
-
-	result := SimulateCombat(player, enemy, defaultCombatPhases)
-	result = injectConsumableEvents(result, selected, len(consumables))
-
-	// Consume used items from inventory
-	for _, c := range selected {
-		if err := removeAdvInventoryItem(c.InventoryID); err != nil {
-			slog.Error("combat: failed to consume item", "id", c.InventoryID, "name", c.Def.Name, "err", err)
-		}
-	}
-
-	p.grantCombatAchievements(userID, result)
-
-	persistDnDHPAfterCombat(userID, result.PlayerStartHP, result.PlayerEndHP)
-	if err := persistDnDPostCombatSubclass(dndChar, playerMods.BerserkerRage, result, playerMods); err != nil {
-		slog.Error("dnd: post-combat subclass persist (arena)", "user", userID, "err", err)
-	}
-
-	if xp := arenaCombatXP(result, monster.ThreatLevel); xp > 0 {
-		if _, err := p.grantDnDXP(userID, xp); err != nil {
-			slog.Error("dnd: grantDnDXP arena", "user", userID, "err", err)
-		}
-	}
-
-	return result, condRepair
-}
-
 // grantCombatAchievements checks combat results for achievement-worthy moments.
 func (p *AdventurePlugin) grantCombatAchievements(userID id.UserID, result CombatResult) {
 	if p.achievements == nil {
@@ -658,15 +565,3 @@ func (p *AdventurePlugin) resolveDungeonAction(
 	return result
 }
 
-// renderArenaCombatFinalMessage builds the post-combat text (rewards, tier progress, etc.)
-// This is sent as the last message after the phase-by-phase combat messages.
-func renderArenaCombatFinalMessage(result CombatResult, monster *ArenaMonster, reward int64, battleXP int, round int) string {
-	if result.PlayerWon {
-		closer := arenaWinCloser(monster.Name, round)
-		return fmt.Sprintf("💀 **%s** has been defeated.\n%s\n🏆 +%d XP | €%d earned",
-			monster.Name, closer, battleXP, reward)
-	}
-	closer := arenaLoseCloser(monster.Name, round)
-	return fmt.Sprintf("The healers are already moving.\n💀 **Defeated.**\n%s\n+%d XP (participation) | Back tomorrow.",
-		closer, arenaParticipationXP)
-}