Fix fruits falling through the floor by replacing single-step physics
with fixed-timestep sub-stepping (16.67ms steps, max 5 per frame).
Add escaped-body cleanup as safety net. Tune physics constants for
better feel (higher density, more bounce, less float).
Update fruit radii to match the community-standard ratios (TomboFry/
moonfloof clones) scaled to our 400px container. Update colors to
match real fruit appearances. Fix "Grapes" → "Grape" per wiki.
