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/core/Music.lua

@@ -368,11 +368,12 @@ function Music.setSurfing(data, surfing)
   if play then Music.play(data, play, nil, { reason = "map" }) end
 end
 
--- battle themes; kind = "wild"|"trainer"|"gym"|"final"
-function Music.playBattle(data, kind, trainerId)
+-- battle themes; kind = "wild"|"trainer"|"gym"|"final".  `song`, when
+-- given, overrides the kind's default -- a mod-set trainer battleTheme.
+function Music.playBattle(data, kind, trainerId, song)
   local b = data.audio and data.audio.battle
   if b then
-    Music.play(data, b[kind] or b.wild, nil,
+    Music.play(data, song or b[kind] or b.wild, nil,
       { reason = "battle", kind = kind, trainerId = trainerId })
   end
 end