Add Allotment: the playable page and its place in the catalogue, on the rules in `allotment/plot.js` #158

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

Done looks like:

• [ ] allotment/index.html draws the beds, the planted crops, and the next packet, and drives it only through allotment/plot.js; it adds no game rule of its own. Tap a bed to plant there, or move with the arrow keys and press Space. Ambient motion only in the crops swaying. The HUD shows the turns left and the score 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. Each crop differs from the others by shape as well as colour. The canvas aria-label says how to play. The rules popup is at most four lines and names the neighbour rules. There is a #mute button and no .mp3 is committed.
• [ ] Listed in GAMES in shell.js as { id: 'allotment', title: 'Allotment', blurb: 'Plant a season of crops where each one scores by its neighbours', kind: 'Strategy', input: 'Tap', accent: '#7a8f3a', 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 allotment that reaches the end line. serve.test.mjs asserts that /allotment/ and /allotment/plot.js answer 200. playtest.test.mjs passes for the folder, and bun test passes.

Files: allotment/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.