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

@@ -0,0 +1,39 @@
+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')