Duck Race rules: a tested `duck-race/stream.js` where you steer a rubber duck across stream lanes past rocks and eddies against three rivals, from plain data, in a two-file pull with a test file that runs in under a second #248

A new Racing game for the catalogue: rubber ducks race down a stream, and you steer yours across lanes around rocks and out of eddies to reach the bridge first. Rules only: no page and no catalogue entry yet.

Done looks like:

• [ ] duck-race/stream.js holds the rules as plain data in, plain data out, with no DOM. start(seed) gives a stated stream length, a stated number of lanes, a seeded layout of rocks, eddies and fast water along it, and your duck plus three rival ducks at the start line; the same seed always gives the same stream and the same rival behaviour.
• [x] tick(state, dt) carries every duck downstream at its lane's current: fast water speeds a duck up by a stated factor, an eddy holds a duck spinning for a stated time, a rock stops a duck until it changes lane. steer(state, -1 | 1) moves your duck one lane, refused past the banks and during a stated cooldown. Rivals change lanes by a seeded rule the test can predict.
• [x] The race ends when every duck has passed the bridge or a stated time limit runs out. A finishing order is reported; first place scores a stated amount, with a stated bonus for never touching a rock. A win line for first place, a loss line otherwise, each with your place.
• [x] duck-race/stream.test.mjs asserts each rule above: the same seed giving the same stream, fast water, an eddy holding a duck, a rock stopping one, steering refused at a bank and in cooldown, the finishing order, the bonus, and the win and loss lines.
• [x] bun test passes for the new file.
• [x] duck-race/stream.test.mjs loads no page and starts no renderer or server: it calls the rules module directly and runs in under one second on its own (bun test duck-race/stream.test.mjs). The pull states the measured time.
• [x] If the check fails only on "test took longer than 600s" or a CPU limit, the pull says so. Do not cut the rules or their tests to get under the limit.

Files: duck-race/stream.js, duck-race/stream.test.mjs — a two-file pull. Risk: none outside the folder. Do not touch shell.js, index.html, town.js, rounds.mjs, forge.json, scripts/, playtest.test.mjs or serve.test.mjs here. Fits: a builder.

Reviewed pull #260 at its head: ticked tick(state, dt) carries every duck downstream at its lane's current: fast water speeds a duck up by a stated factor, an eddy holds a duck spinning for a stated time, a rock stops a duck until it changes lane. steer(state, -1 | 1) moves your duck one lane, refused past the banks and during a stated cooldown. Rivals change lanes by a seeded rule the test can predict.; The race ends when every duck has passed the bridge or a stated time limit runs out. A finishing order is reported; first place scores a stated amount, with a stated bonus for never touching a rock. A win line for first place, a loss line otherwise, each with your place.; duck-race/stream.test.mjs asserts each rule above: the same seed giving the same stream, fast water, an eddy holding a duck, a rock stopping one, steering refused at a bank and in cooldown, the finishing order, the bonus, and the win and loss lines.; bun test passes for the new file.; duck-race/stream.test.mjs loads no page and starts no renderer or server: it calls the rules module directly and runs in under one second on its own (bun test duck-race/stream.test.mjs). The pull states the measured time.; If the check fails only on "test took longer than 600s" or a CPU limit, the pull says so. Do not cut the rules or their tests to get under the limit..

Left open after pull #260 landed: 1 unticked item remains (duck-race/stream.js holds the rules as plain data in, plain data out, with no DOM. start(seed) gives a stated stream length, a stated number of lanes, a seeded layout of rocks, eddies and fast water along it, and your duck plus three rival ducks at the start line; the same seed always gives the same stream and the same rival behaviour.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.