Add Weathervane: the playable page and its place in the catalogue, on the rules in `weathervane/vanes.js` #374

Make the Weathervane rules playable and listed, in the same shape as apple-bobbing/. Starts once the Weathervane rules issue has landed weathervane/vanes.js.

Done looks like:

• [ ] weathervane/index.html draws the row of rooftops with each vane at its direction, the links between them and the target wind on a windsock. Arrow keys pick a vane, Space or Enter or a tap turns it, U undoes, R restarts. It drives the game only through the export of vanes.js and adds no game rule of its own. The HUD reads from the rules state, and every move, refusal and end line is announced through an aria-live region.
• [ ] Follows the README Visual rules: 360x280 canvas via engine.js, fixed 30-degree camera, hard shadows screen-right, ambient motion (clouds) held still under prefers-reduced-motion. 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 with id weathervane, title Weathervane, kind Puzzle, input Tap, music: false and forged: true; the matching <li> in the root index.html; 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 weathervane that reaches the win line (the first level's solution is in the rules), serve.test.mjs asserts /weathervane/ and /weathervane/vanes.js answer 200, a weathervane/page.test.mjs stays inside the per-game test budget, playtest.test.mjs passes for the folder, and bun test passes. bun scripts/design-lint.ts passes.

Files: weathervane/index.html, weathervane/page.test.mjs, shell.js, index.html, README.md, rounds.mjs, serve.test.mjs, og/. Risk: shared files with every other page pull; rebase before pushing. Fits: a builder.