feat(audio): looping per-season background music system

MusicService (looping audioplayers player, independent of SFX) driven by the
active season theme's new 'bgm' key; switches track on season change, pauses on
app background, all failures swallowed. Separate Music on/off toggle in Settings
(persisted, independent of SFX). Season packs carry bgm keys (menu/season_001/
season_002), manifest regenerated. Assets slot assets/audio/bgm/ ready — drop in
menu.mp3/season_001.mp3/season_002.mp3 (CC0) and it plays; silent until then.
180 tests green, analyze clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/audio/bgm/README.txt

@@ -0,0 +1,7 @@
+Drop background-music tracks here as MP3, named by theme key:
+  menu.mp3        — home/menu (SeasonTheme.fallback, bgm="menu")
+  season_001.mp3  — Season 1 "First Bloom"
+  season_002.mp3  — Season 2 "Summer Tide"
+
+Use CC0 / royalty-free, commercial-safe tracks (see docs). The app plays
+whatever is present and stays silent (no error) for any missing track.

assets/seasons/season_001/pack.json

@@ -8,7 +8,8 @@
   },
   "theme": {
     "tileSet": "spring",
-    "background": "background.webp"
+    "background": "background.webp",
+    "bgm": "season_001"
   },
   "stages": [
     {