- Reduce pushSpeed across all 20 levels to ~60% of original values in
tools/genZuma.js, making the chain advance more slowly and giving the
player more reaction time. starScores() thresholds auto-adjust downward
since they are derived from pushSpeed.
- Add per-level background art (zuma-background-1 / -2) alternated per
level via a new `background` field in each level definition; draw it in
ZumaGame.drawBackground() over the solid fallback rectangle.
- Register the two new background textures and sfx-zuma-lose in the asset
manifest.
- Add playLoseFanfare() mirroring playStartFanfare() to duck/pause music
and play the lose SFX when the chain goes over the edge (onLost).
- Add a black stroke to the "ZUMA!" win banner for readability.
- Bump music volume to 0.5 in zuma-music.json.
