Add Dinghy Race: the playable page and its place in the catalogue, on the rules already in `dinghy-race/dinghy.js` #110

A new game in the house style, mounted like the other forged games, built on the tested rules in dinghy-race/dinghy.js. This is the second half of #92.

Done looks like:

• [ ] dinghy-race/index.html draws the game and drives it only through dinghy-race/dinghy.js; it adds no game rule of its own. Dragging along the sheet trims the sail (Up and Down trim in and out). Ambient motion only in the water ripples and the start-line flags. The HUD and end line show what the rules state reports, with the win and loss lines the rules name.
• [ ] Follows the README Visual rules: 360x280 canvas, fixed 30-degree camera, hard shadows screen-right, ambient motion stopped under prefers-reduced-motion. Keyboard and touch or mouse both work; the canvas aria-label says how to play; the rules popup is at most four lines; a #mute button and no .mp3 committed.
• [ ] Listed in GAMES in shell.js as { id: 'dinghy-race', title: 'Dinghy Race', blurb: 'Drag to trim the sail to the wind and round three buoys ahead of the fleet', kind: 'Racing', input: 'Drag', accent: '#2e8a7a', music: false, forged: true }, with the matching <li> in the root index.html (same data-kind, data-input, --accent and tags line; serve.test.mjs checks they match), in the README Games table and the README list of games without music. bun scripts/og.mjs has been run so og/dinghy-race.png and its share tags exist.
• [ ] rounds.mjs has a scripted round for dinghy-race that reaches the end line; serve.test.mjs asserts /dinghy-race/ and /dinghy-race/dinghy.js answer 200; playtest.test.mjs passes for the folder (played differs from untouched in picture and HUD, by pointer and by keyboard alone), and bun test passes.

Files: dinghy-race/index.html, shell.js, index.html, README.md, serve.test.mjs, rounds.mjs, og/. Risk: one-line conflicts with the other new-game pulls in shell.js, index.html and the README, so append and rebase on main before pushing. If a test unrelated to this game times out under the full suite, say so in the pull and do not change other games. Fits: a builder.