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_narrative_test.go

@@ -106,27 +106,6 @@ func TestRenderCombatLog_LossPhaseMessages(t *testing.T) {
 	}
 }
 
-func TestRenderCombatLogArena_ProducesPhaseMessages(t *testing.T) {
-	result := CombatResult{
-		PlayerWon:     true,
-		PlayerStartHP: 100,
-		EnemyStartHP:  60,
-		PlayerEndHP:   70,
-		EnemyEndHP:    0,
-		TotalRounds:   3,
-		Closeness:     0.3,
-		Events: []CombatEvent{
-			{Round: 1, Phase: "opening", Actor: "player", Action: "hit", Damage: 30, EnemyHP: 30, PlayerHP: 100},
-			{Round: 2, Phase: "clash", Actor: "player", Action: "hit", Damage: 30, EnemyHP: 0, PlayerHP: 70},
-		},
-	}
-
-	msgs := RenderCombatLogArena(result, "Hero", "Ratticus")
-	if len(msgs) < 1 {
-		t.Fatalf("expected at least 1 phase message, got %d", len(msgs))
-	}
-}
-
 func TestRenderCombatLog_ConsumableEvents(t *testing.T) {
 	result := CombatResult{
 		PlayerWon:     true,