Snail Race page: `snail-race/index.html` on the landed `snail-race/race.js` rules, listed in the catalogue and the town #266

A new game in the house style, mounted like the other forged games, built on the tested rules in snail-race/race.js.

Done looks like:

• [ ] snail-race/index.html draws the garden lane from the fixed camera, four snails with their slime trails, the beat marker and the finish line, and drives it only through snail-race/race.js; it adds no game rule of its own. Space or tap steps, B or a second-finger tap bursts. Ambient motion only in grass at the lane edge. The HUD shows slime, place and time as the rules state reports them.
• [ ] Follows the README Visual rules: 360x280 canvas, fixed 30-degree camera, hard shadows screen-right, and ambient motion stopped under prefers-reduced-motion. Your snail is marked by a shell pattern as well as colour, and the beat window is shown, not only heard. The canvas aria-label says how to play. The rules popup is at most four lines. There is a #mute button and no .mp3 is committed.
• [ ] Listed in GAMES in shell.js as { id: 'snail-race', title: 'Snail Race', blurb: 'Pace your snail to the beat and spend slime on bursts to win the lane', kind: 'Racing', input: 'Keys', accent: '#7fa35a', music: false, forged: true }. It has the matching <li> in the root index.html, a building in the town from town.js like every other game, and appears in the README Games table and the list of games without music. bun scripts/og.mjs has been run.
• [ ] rounds.mjs has a scripted round for snail-race that reaches the end line. serve.test.mjs asserts that /snail-race/ and /snail-race/race.js answer 200. playtest.test.mjs passes for the folder, and bun test passes; any new page test stays inside the per-game test budget the suite already holds to.

Files: snail-race/index.html, shell.js, index.html, town.js if buildings are listed there, README.md, serve.test.mjs, rounds.mjs, og/. Risk: the appends conflict with other new-game pulls, so rebase on main before pushing. Fits: a builder.

The rules are on main in 75183a4 (snail-race/race.js): read its header for the stated constants and calls (tick, step, burst). Replaces #203 and #263, parked only because an issue they once waited on was split; the rules they need are on main.