Honor trainer battleTheme override (fixes #945)

trainers.battleTheme validated and merged onto the trainer record but was
never read: battle music came solely from data.audio.battle[kind] where
kind is computeMusicKind()'s final/gym/trainer/wild.  Route both battle-
theme start sites through a single choke point:

- BattleState:playBattleTheme() cues Music.playBattle with the override
  (self.trainer.battleTheme via battleTheme()), defaulting to the kind
  when unset, so vanilla fights and #782's non-gym Giovanni are unchanged.
- BattleState:enter() and OverworldController:pushBattle() both call it.
- Music.playBattle gains an optional 4th song arg that overrides the kind
  default, and real call sites now populate the music.select trainerId.
- Victory jingles stay kind-based: a custom battle theme has no derivable
  win-variant.

New ROM-free T2 suite tests/engine/trainer_battle_theme_bug945.lua covers
mod load, override resolution, the choke point, and the nil-override
parity gate.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

src/world/OverworldController.lua

@@ -760,8 +760,8 @@ function OverworldState:pushBattle(battle)
   local enemyLevel = battle.enemy and battle.enemy.mon and battle.enemy.mon.level or 0
   -- the battle theme starts with the wipe, not after it
   -- (audio/play_battle_music.asm runs before the transition)
-  if battle.computeMusicKind then
-    require("src.core.Music").playBattle(Game.data, battle:computeMusicKind())
+  if battle.playBattleTheme then
+    battle:playBattleTheme()
   end
 
   -- The fade back in from white on the way out is BattleState:finish()'s