Add Bumper Cars: the playable page and its place in the catalogue, on the rules in `bumper-cars/floor.js` #345

A new game in the house style, mounted like the other forged games. It is built on the tested rules in bumper-cars/floor.js, landed in 8728d37.

Done looks like:

• bumper-cars/index.html draws the walled fairground floor of the rules' size with your car and the three seeded rivals, each pointing along its heading. Left and right arrow keys (or A and D) steer, and on touch, the left and right halves of the canvas steer. The floor ticks on at a steady stated pace via step. The HUD shows the score, lives, rivals still on the floor and ticks left, all read from the rules state. A shove, a rival into the wall and a head-on bump are each announced. R restarts. It drives the game only through BumperCars from floor.js and adds no game rule of its own. Each move, each refusal reason and the end line are announced through an aria-live region.
• Follows the README Visual rules: 360x280 canvas via engine.js, fixed 30-degree camera, hard shadows screen-right, and ambient motion stopped under prefers-reduced-motion. Your car differs from the rivals by shape as well as colour, and each car's heading is clear from its shape without 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 with id bumper-cars, title Bumper Cars, kind Arcade, input Keys, music: false and forged: true. It has the matching <li> in the root index.html and a building in the town from town.js like every other game. It 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 bumper-cars that reaches the end line. serve.test.mjs asserts that /bumper-cars/ and /bumper-cars/floor.js answer 200. playtest.test.mjs passes for the folder, and bun test passes. Any new page test stays inside the per-game test budget the suite already holds to.

Files: bumper-cars/index.html, shell.js, index.html, town.js, README.md, serve.test.mjs, rounds.mjs, og/. Risk: these appends conflict with other new-game pulls in flight (Pumpkin Patch, Ticket Booth and the other pages in this plan), so rebase on main before pushing. Do not change the rules file; if it lacks something the page needs, say so in the pull. Fits: a builder.

Reviewed pull #351 at its head: ticked bumper-cars/index.html draws the walled fairground floor of the rules' size with your car and the three seeded rivals, each pointing along its heading. Left and right arrow keys (or A and D) steer, and on touch, the left and right halves of the canvas steer. The floor ticks on at a steady stated pace via step. The HUD shows the score, lives, rivals still on the floor and ticks left, all read from the rules state. A shove, a rival into the wall and a head-on bump are each announced. R restarts. It drives the game only through BumperCars from floor.js and adds no game rule of its own. Each move, each refusal reason and the end line are announced through an aria-live region.; Follows the README Visual rules: 360x280 canvas via engine.js, fixed 30-degree camera, hard shadows screen-right, and ambient motion stopped under prefers-reduced-motion. Your car differs from the rivals by shape as well as colour, and each car's heading is clear from its shape without 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 with id bumper-cars, title Bumper Cars, kind Arcade, input Keys, music: false and forged: true. It has the matching <li> in the root index.html and a building in the town from town.js like every other game. It 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 bumper-cars that reaches the end line. serve.test.mjs asserts that /bumper-cars/ and /bumper-cars/floor.js answer 200. playtest.test.mjs passes for the folder, and bun test passes. Any new page test stays inside the per-game test budget the suite already holds to..