Add Rain Barrel: the playable page and its place in the catalogue, on the rules in `rain-barrel/gutter.js` #220

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

Done looks like:

• [x] rain-barrel/index.html draws the cottage wall with its row of gutter spouts, falling drops, the barrel and the yard puddle from the fixed camera and drives it only through rain-barrel/gutter.js; it adds no game rule of its own. Tap a spout or press left and right to move the barrel; tap the barrel to empty it. Ambient motion only in clouds drifting over the roof. The HUD shows time left, barrel fill, puddle level and score as the rules state reports them.
• [x] Follows the README Visual rules: 360x280 canvas, fixed 30-degree camera, hard shadows screen-right, and ambient motion stopped under prefers-reduced-motion. The puddle limit is marked with a line and a full barrel with a lid icon, not only a colour. 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.
• [x] Listed in GAMES in shell.js as { id: 'rain-barrel', title: 'Rain Barrel', blurb: 'Slide the barrel under each spout before the yard floods', kind: 'Catching', input: 'Tap', accent: '#5f86a3', 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.
• [x] rounds.mjs has a scripted round for rain-barrel that reaches the end line. serve.test.mjs asserts that /rain-barrel/ and /rain-barrel/gutter.js answer 200. playtest.test.mjs passes for the folder, and bun test passes.

Files: rain-barrel/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.

Reviewed pull #246 at its head: ticked rain-barrel/index.html draws the cottage wall with its row of gutter spouts, falling drops, the barrel and the yard puddle from the fixed camera and drives it only through rain-barrel/gutter.js; it adds no game rule of its own. Tap a spout or press left and right to move the barrel; tap the barrel to empty it. Ambient motion only in clouds drifting over the roof. The HUD shows time left, barrel fill, puddle level and score 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 puddle limit is marked with a line and a full barrel with a lid icon, not only a colour. 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: 'rain-barrel', title: 'Rain Barrel', blurb: 'Slide the barrel under each spout before the yard floods', kind: 'Catching', input: 'Tap', accent: '#5f86a3', 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 rain-barrel that reaches the end line. serve.test.mjs asserts that /rain-barrel/ and /rain-barrel/gutter.js answer 200. playtest.test.mjs passes for the folder, and bun test passes..