Add Sack Race: the playable page and its place in the catalogue, on the rules in `sack-race/course.js` #308

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

Done looks like:

• [ ] sack-race/index.html draws a grass lane with bunting and four hopping sacks, the finish tape, and a small landing marker that shows the hop window closing. Space, tap or click hops. The HUD shows the distance left and the current place. The page drives the game only through sack-race/course.js and adds no game rule of its own; each event the rules report is announced through an aria-live region.
• [ ] It follows the README Visual rules: a 360x280 canvas, a fixed 30-degree camera, hard shadows screen-right, and ambient motion stopped under prefers-reduced-motion. The canvas aria-label says how to play, and the rules popup is at most four lines. There is a #mute button, and no .mp3 is committed.
• [ ] GAMES in shell.js lists it as { id: 'sack-race', title: 'Sack Race', blurb: 'Hop in time with your landing and beat three rivals to the tape', kind: 'Racing', input: 'Tap', accent: '#c2703d', music: false, forged: true }. It has the matching <li> in the root index.html, a place in the README Games table and the list of games without music, and its town building from the GAMES entry. bun scripts/og.mjs has been run.
• [ ] rounds.mjs has a scripted round for sack-race that reaches the end line. serve.test.mjs asserts that /sack-race/ and /sack-race/course.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: sack-race/index.html, shell.js, index.html, README.md, serve.test.mjs, rounds.mjs, og/. Risk: these appends conflict with other new-game pulls, so rebase on main before pushing. Fits: a builder.

Waits for the rules issue to land on main.