Add Stone Skip: skim a flat stone across a lake, count the bounces #4

A sixth game in the house style: one small lakeshore diorama floating on white, one readable physical action.

Done looks like:

• [ ] stone-skip/index.html exists, loads ../shell.js, calls PixelPlays.mount({ id: 'stone-skip', rules }), sets window.PP_SCENE and loads ../engine.js, following balote/index.html.
• [ ] Drag back and release sets angle and power (arrows + space as keyboard fallback); the stone skips across the water in whole pixels with a ripple per bounce, and the HUD shows bounces this throw and the best of five throws.
• [ ] Too steep or too slow sinks the stone on the first touch; a flat fast throw skips more. The rules popup says so in at most four lines.
• [ ] Added to GAMES in shell.js and to the Games table in README.md; it has a #mute button and plays with no .mp3 (no track is committed: MUSIC.md tracks are generated outside the repo).
• [ ] serve.test.mjs asserts /stone-skip/ answers 200 text/html (the track check stays on the five games that have one); bun test passes.
• [ ] Follows the README Visual rules: 360x280 canvas, 30-degree camera, hard shadows screen-right, reed/grass sway stops under prefers-reduced-motion.

Files: stone-skip/index.html, shell.js, README.md, serve.test.mjs. Risk: merge conflicts in GAMES/README/test with the other new-game issues: append one line each, rebase on main. Fits: a builder.