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_bossflow_test.go

@@ -84,30 +84,6 @@ func TestArenaBossPhaseTwoAt(t *testing.T) {
 	}
 }
 
-// Phase L2 step 4b — flag gate. Empty / "0" / "false" disable the new
-// boss-flow path; anything else (including "1") enables it.
-func TestArenaBossFlowEnabled(t *testing.T) {
-	cases := []struct {
-		val  string
-		want bool
-	}{
-		{"", false},
-		{"0", false},
-		{"false", false},
-		{"FALSE", false},
-		{"False", false},
-		{"1", true},
-		{"true", true},
-		{"on", true},
-	}
-	for _, c := range cases {
-		t.Setenv("ARENA_BOSS_FLOW", c.val)
-		if got := arenaBossFlowEnabled(); got != c.want {
-			t.Errorf("arenaBossFlowEnabled with %q = %v want %v", c.val, got, c.want)
-		}
-	}
-}
-
 // Phase L2 step 4b — staged-narration assembly. The intro line leads,
 // followed by the combat-log phases, mirroring streamOrSend's
 // intro+phases pattern in dnd_zone_cmd.go.