Add Lily Pond: the playable page and its place in the catalogue, on the rules in `lily-pond/pond.js` #167

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

Done looks like:

• [ ] lily-pond/index.html draws the water, the pads, the sinking ripple and the frog, and drives it only through lily-pond/pond.js; it adds no game rule of its own. Swipe or tap beside the frog to hop that way, or use the arrow keys. Ambient motion only in the water rippling. The HUD shows the level, pads left and retries the rules report.
• [ ] Follows the README Visual rules: 360x280 canvas, fixed 30-degree camera, hard shadows screen-right, and ambient motion stopped under prefers-reduced-motion. The frog and the pads stand out from the water by shape as well as colour, and a sunk pad is gone, not dimmed. 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: 'lily-pond', title: 'Lily Pond', blurb: 'Hop the frog onto every lily pad once before they sink', kind: 'Puzzle', input: 'Keys', accent: '#2f7a6a', 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 lily-pond that reaches the end line. serve.test.mjs asserts that /lily-pond/ and /lily-pond/pond.js answer 200. playtest.test.mjs passes for the folder, and bun test passes.

Files: lily-pond/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.