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

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

js/solutions/model.js

@@ -0,0 +1,51 @@
+// m02 model: the data the whole game is built from. Plain objects, never mutated.
+
+/** @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. */
+export const point = (x, y) => ({ x, y })
+
+/** Structural equality: same x and same y. */
+export const pointEquals = (a, b) => a.x === b.x && a.y === b.y
+
+const deltas = {
+  Up: point(0, -1),
+  Down: point(0, 1),
+  Left: point(-1, 0),
+  Right: point(1, 0),
+}
+
+/** The unit step for a direction (y grows downward). */
+export const delta = (dir) => deltas[dir]
+
+const opposites = { Up: 'Down', Down: 'Up', Left: 'Right', Right: 'Left' }
+
+/** The direction pointing the other way. */
+export const opposite = (dir) => opposites[dir]
+
+/** Everything that can happen to the game, as data. `Turn` is a function because it carries a direction. */
+export const Action = {
+  Tick: { tag: 'Tick' },
+  Turn: (dir) => ({ tag: 'Turn', dir }),
+  TogglePause: { tag: 'TogglePause' },
+  Restart: { tag: 'Restart' },
+  Frame: { tag: 'Frame' },
+}
+
+/** A fresh 20x20 game: 3-segment snake heading Right, no apple yet. */
+export const initialState = (seed) => ({
+  width: 20,
+  height: 20,
+  snake: [point(10, 10), point(9, 10), point(8, 10)],
+  dir: 'Right',
+  nextDir: 'Right',
+  apple: null,
+  seed,
+  score: 0,
+  status: 'Playing',
+  frame: 0,
+})