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/mods/Schemas.lua

@@ -577,6 +577,9 @@ R.trainers = {
     aiMods = f.opt(f.any),
     aiClass = f.opt(f.id("ai_classes")),
     brain = f.opt(f.fn),
+    -- Per-trainer battle theme (an audio.songs id): overrides the
+    -- kind-based default (wild/trainer/gym/final) for this trainer's
+    -- battles.  The victory jingle stays kind-based.
     battleTheme = f.opt(f.id("music")),
   },
   example = 'mod.content.trainers:patch("OPP_BROCK", { baseMoney = 99 })',