Introduce a new battle background spritesheet (advancewars-battlebg) that
displays context-appropriate backdrops behind each fighter during battle
cut-ins. The backdrop is selected based on the terrain/building type the
fighter is standing on (city, base, airport, port, plain, wood, mountain,
sea, reef, shoal, river, bridge, road, hq).
- Add battleBgSheet to artwork.json and asset manifest
- Implement playBattleAnim() backdrop system with terrain lookup
- Provide procedural fallback matching the real sheet's frame map for
art-free playability
- Mirror defender backdrops to match mirrored defender sprites
- Document the 14-frame sheet spec in sprites.md (256×256 cells, 5 cols)
