Add Tower Duel: the playable page and its place in the catalogue, on the rules in `tower-duel/duel.js` #131

Second half of #102. A new game in the house style, mounted like the other forged games, built on the tested rules in tower-duel/duel.js.

Done looks like:

• [ ] tower-duel/index.html draws the field and drives it only through tower-duel/duel.js. It adds no game rule of its own. To play, tap a card and then tap your half; or press 1–4 to pick, arrows to move a ring, and Space to play. The HUD shows elixir, hand, clock and towers down. The end line shows what the rules report, with a win, loss or draw line.
• [ ] 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: 'tower-duel', title: 'Tower Duel', blurb: 'Tap a card, tap your half: send troops over the bridges and topple the rival king tower', kind: 'Strategy', input: 'Tap', accent: '#8a3b3b', music: false, forged: true }. It has the matching <li> in the root index.html, which serve.test.mjs checks, 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 tower-duel that reaches the end line. serve.test.mjs asserts that /tower-duel/ and /tower-duel/duel.js answer 200. playtest.test.mjs passes for the folder by pointer and by keyboard alone, and bun test passes.

Files: tower-duel/index.html, shell.js, index.html, README.md, serve.test.mjs, rounds.mjs, og/. Risk: a 90-second match makes a slow playtest. The scripted round may idle with g.step until the clock ends it. Rebase on main before pushing; the appends conflict with other new-game pulls. Fits: a builder.