Add Lock Keeper: the playable page and its place in the catalogue, on the rules in `lock-keeper/lock.js` #141

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

Done looks like:

• [ ] lock-keeper/index.html draws the flight of locks, the boats and the water, and drives them only through lock-keeper/lock.js; it adds no game rule of its own. Tap a paddle or gate to work it and tap a boat to move it; or use Left and Right to pick a lock and the number keys for its paddles and gates. A refused operation shows the reason the rules give. Ambient motion only in the water surface. The HUD shows operations and par 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. 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: 'lock-keeper', title: 'Lock Keeper', blurb: 'Work the paddles and gates to bring every narrowboat through the flight of locks', kind: 'Strategy', input: 'Tap', accent: '#5a6e8a', music: false, forged: true }. It has the matching <li> in the root index.html, 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 lock-keeper that reaches the end line. serve.test.mjs asserts that /lock-keeper/ and /lock-keeper/lock.js answer 200. playtest.test.mjs passes for the folder, and bun test passes.

Files: lock-keeper/index.html, shell.js, index.html, 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.