feat(js): m01 util and m02 model with tests, fixtures, purity audit

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

js/src/model.js

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+import { todo } from './todo.js'
+
+/** @typedef {{x: number, y: number}} Point */
+/** @typedef {'Up'|'Down'|'Left'|'Right'} Direction */
+/** @typedef {'Playing'|'Paused'|'GameOver'|'Won'} Status */
+/** @typedef {{width: number, height: number, snake: Point[], dir: Direction, nextDir: Direction,
+ *   apple: Point|null, seed: number, score: number, status: Status, frame: number}} GameState */
+
+/** A grid cell `{x, y}`. */
+export const point = (x, y) => todo('m02')
+
+/** Structural equality: same x and same y. */
+export const pointEquals = (a, b) => todo('m02')
+
+/** The unit step for a direction: Up {0,-1}, Down {0,1}, Left {-1,0}, Right {1,0} (y grows downward). */
+export const delta = (dir) => todo('m02')
+
+/** The direction pointing the other way. */
+export const opposite = (dir) => todo('m02')
+
+/**
+ * Everything that can happen to the game, as data:
+ * `Tick`, `Turn(dir)`, `TogglePause`, `Restart`, `Frame`, each `{ tag, ... }`.
+ * `Turn` is a function because it carries a direction; the rest are constants
+ * (the getters below only exist so the stub compiles; replace them with plain values).
+ */
+export const Action = {
+  get Tick() { return todo('m02') },
+  Turn: (dir) => todo('m02'),
+  get TogglePause() { return todo('m02') },
+  get Restart() { return todo('m02') },
+  get Frame() { return todo('m02') },
+}
+
+/**
+ * A fresh game: 20x20, snake [(10,10),(9,10),(8,10)] heading Right (dir and nextDir),
+ * apple null, the given seed, score 0, status 'Playing', frame 0.
+ */
+export const initialState = (seed) => todo('m02')

js/src/todo.js

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+// Placeholder body for unimplemented stubs: replace each `todo('mNN')` call with real code.
+export const todo = (module) => {
+  throw new Error(`TODO: implement in ${module}`)
+}

js/src/util.js

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+import { todo } from './todo.js'
+
+/** `range(n)` is `[0, 1, ..., n-1]`; `range(0)` is `[]`. */
+export const range = (n) => todo('m01')
+
+/** `xs` without its last element; an empty list stays empty. Never mutates `xs`. */
+export const dropLast = (xs) => todo('m01')
+
+/** True if some element `x` of `xs` satisfies `eq(x, p)`. `eq` lets callers compare structurally (e.g. `pointEquals`). */
+export const contains = (xs, p, eq) => todo('m01')
+
+/** `pipe(x, f, g)` is `g(f(x))`: thread a value through functions left to right. */
+export const pipe = (x, ...fns) => todo('m01')
+
+/** `compose(f, g)(x)` is `f(g(x))`: functions applied right to left. */
+export const compose = (...fns) => todo('m01')
+
+/** Freeze `obj` and everything reachable from it, then return it (same object). */
+export const deepFreeze = (obj) => todo('m01')