feat(js): m04 random and spawn with fixtures and tests

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

js/src/random.js

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+import { todo } from './todo.js'
+
+/**
+ * A `Random<A>` is a plain function from a seed to a pair: `seed => [value, nextSeed]`.
+ * Seeds and values are unsigned 32-bit integers. Nothing is hidden or mutated: the caller carries the seed.
+ */
+
+/**
+ * The generator (mulberry32). With `s = (seed + 0x6D2B79F5) mod 2^32`:
+ *   t = imul(s ^ (s >>> 15), s | 1)
+ *   t = t ^ (t + imul(t ^ (t >>> 7), t | 61))
+ *   value = (t ^ (t >>> 14)) >>> 0
+ * Returns `[value, s]`: the new seed is `s`.
+ */
+export const nextUint32 = (seed) => todo('m04')
+
+/** A Random that always gives `a` and leaves the seed untouched. */
+export const pure = (a) => todo('m04')
+
+/** Transform the value a Random produces; the seed threading is unchanged. */
+export const map = (r, f) => todo('m04')
+
+/** Run `r`, feed its value to `f` to pick the next Random, and run that with the seed `r` left behind. */
+export const flatMap = (r, f) => todo('m04')
+
+/** A Random giving a number in `[0, n)`: `nextUint32` value modulo `n`. */
+export const intBelow = (n) => todo('m04')
+
+/** Run a Random from a seed: `[value, nextSeed]`. */
+export const run = (r, seed) => todo('m04')

js/src/spawn.js

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+import { todo } from './todo.js'
+
+/** Every board cell not covered by the snake, row-major (y outer loop, x inner loop). The apple does not count as occupied. */
+export const freeCells = (state) => todo('m04')
+
+/**
+ * Put an apple on a random free cell, drawing from `state.seed` (use `intBelow` and store the new seed).
+ * Unchanged if there already is an apple or the status is not 'Playing'.
+ * If no cell is free the board is full: status becomes 'Won' (seed unchanged, no apple).
+ */
+export const spawnApple = (state) => todo('m04')
+
+/** One game step: `advance`, then make sure there is an apple. */
+export const tick = (state) => todo('m04')
+
+/** A fresh game (`initialState`) with its first apple already placed. */
+export const newGame = (seed) => todo('m04')