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/adventure_arena_combat.go

@@ -1,36 +1,3 @@
 package plugin
 
-import (
-	"fmt"
-	"math/rand/v2"
-)
-
 const arenaParticipationXP = 60
-
-// ── Closer Lines ───────────────────────────────────────────────────────────
-
-func arenaWinCloser(loserName string, lastRound int) string {
-	closers := []string{
-		"%s fought. It counts.",
-		"%s will be back. The arena keeps score.",
-		fmt.Sprintf("%%s has until tomorrow to think about round %d.", lastRound),
-		"%s gave you more trouble than you'd like to admit. They don't need to know that.",
-		"%s loses this one. The next one is an open question.",
-		"%s came here to fight and did. The result is a separate matter.",
-		"%s is already planning the rematch. You can feel it.",
-	}
-	return fmt.Sprintf(closers[rand.IntN(len(closers))], loserName)
-}
-
-func arenaLoseCloser(winnerName string, lastRound int) string {
-	closers := []string{
-		"You fought. It counts.",
-		"You'll be back. The arena keeps score.",
-		fmt.Sprintf("You have until tomorrow to think about round %d.", lastRound),
-		fmt.Sprintf("You gave %s more trouble than they'd like to admit. Small comfort. Still comfort.", winnerName),
-		"You lose this one. The next one is an open question.",
-		"You came here to fight and did. The result is a separate matter.",
-		fmt.Sprintf("%s won this one. You're already planning the rematch.", winnerName),
-	}
-	return closers[rand.IntN(len(closers))]
-}