Milk Churns rules: a tested `milk-churns/churns.js` where you pour milk between three churns to leave an exact amount within par, in a two-file pull with a test file that runs in under a second #353

This adds the Milk Churns rules in milk-churns/churns.js and milk-churns/churns.test.mjs, and nothing else. bun test milk-churns/churns.test.mjs passes: 11 tests, 412 assertions, 43 ms (under the one-second limit).

What changed

• Levels: there are seven in LEVELS, each with a name, three capacities, the starting fill (largest churn full, the others empty), a target in litres and a stated solution. Par is the solution's length. Every solution is a shortest one, found by breadth-first search, and par runs from 2 to 8 pours.
• Functions: start, pour, undo, restart and describe are plain data in, plain data out, with no DOM, timers or randomness. The module exports like pumpkin-patch/plots.js: module.exports, or a MilkChurns global for the page.
• Pouring: a pour moves min(source, target space) litres, so the total milk never changes. The level is won when any churn holds exactly the target.
• Refusals: each returns the same state and a reason: unknown-churn, same-churn, empty-source, full-target, nothing-to-undo or game-over.
• Undo after a win: undo is still allowed once the level is won, as in Pumpkin Patch.
• Win line: it reads like "You win: 2 L left in a churn in 2 pours against par 2, at par", with "under par" or "N over par" otherwise.

Tests: besides the checks you listed, the tests check that every level's starting fill is the largest churn full and that start falls back to level 0 for an unknown level.

Not done

• No page and no catalogue entry, as the issue says.
• The "under par" win line is tested on a state whose par I raised by hand. Every solution is already a shortest one, so no real level can be won under par.

Checks not run

• scripts/gate.ts: it doesn't exist in this clone, so the gate didn't run.
• bun x tsc --noEmit: it only printed its help text because the clone has no tsconfig, so it checked nothing.

To try it: run bun test milk-churns/churns.test.mjs.

Closes #332

This adds the Milk Churns rules in milk-churns/churns.js and milk-churns/churns.test.mjs, and nothing else. bun test milk-churns/churns.test.mjs passes: 11 tests, 412 assertions, 43 ms (under the one-second limit).

What changed

• Levels: there are seven in LEVELS, each with a name, three capacities, the starting fill (largest churn full, the others empty), a target in litres and a stated solution. Par is the solution's length. Every solution is a shortest one, found by breadth-first search, and par runs from 2 to 8 pours.
• Functions: start, pour, undo, restart and describe are plain data in, plain data out, with no DOM, timers or randomness. The module exports like pumpkin-patch/plots.js: module.exports, or a MilkChurns global for the page.
• Pouring: a pour moves min(source, target space) litres, so the total milk never changes. The level is won when any churn holds exactly the target.
• Refusals: each returns the same state and a reason: unknown-churn, same-churn, empty-source, full-target, nothing-to-undo or game-over.
• Undo after a win: undo is still allowed once the level is won, as in Pumpkin Patch.
• Win line: it reads like "You win: 2 L left in a churn in 2 pours against par 2, at par", with "under par" or "N over par" otherwise.

Tests: besides the checks you listed, the tests check that every level's starting fill is the largest churn full and that start falls back to level 0 for an unknown level.

Not done

• No page and no catalogue entry, as the issue says.
• The "under par" win line is tested on a state whose par I raised by hand. Every solution is already a shortest one, so no real level can be won under par.

Checks not run

• scripts/gate.ts: it doesn't exist in this clone, so the gate didn't run.
• bun x tsc --noEmit: it only printed its help text because the clone has no tsconfig, so it checked nothing.

To try it: run bun test milk-churns/churns.test.mjs.

Left #332 open: 1 unticked item remain (milk-churns/churns.test.mjs asserts each rule above. It checks that each level's stated solution wins within par, that a pour stops at an empty source and at a full target, and that total milk is conserved after every pour. It also checks each refusal with its reason, that undo restores the exact previous state, the win line at, under and over par, and restart.).