SNAKE game v0.3: round screen, swipe control

- 16x16 grid, 320px board centered on round 466 screen, designWidth 466
- pure snake logic in src/common/snake.js (step/turn/rotateDir/spawnFood, TDD'd)
- for-directive segment rendering, show-gated game-over overlay, 300ms tick
- swipe direction input (crown rotation confirmed unsupported on Vela S4)
- tests: snake logic + page wiring

src/common/snake.js

@@ -0,0 +1,59 @@
+// Snake core — pure functions, no Vela APIs. Exports: initialSnake, step, turn, spawnFood.
+function initialSnake() {
+  return [
+    { x: 6, y: 7 },
+    { x: 5, y: 7 },
+    { x: 4, y: 7 }
+  ]
+}
+
+// Advance the snake one cell in `dir` ({x,y} unit vector). Returns { snake, ate, alive }.
+function step(snake, dir, food, N) {
+  const head = snake[0]
+  const nx = head.x + dir.x
+  const ny = head.y + dir.y
+  if (nx < 0 || ny < 0 || nx >= N || ny >= N) {
+    return { snake, ate: false, alive: false }
+  }
+  const ate = (nx === food.x && ny === food.y)
+  // tail cell vacates unless we grew, so exclude it from self-collision check
+  const body = ate ? snake : snake.slice(0, snake.length - 1)
+  for (let i = 0; i < body.length; i++) {
+    if (body[i].x === nx && body[i].y === ny) {
+      return { snake, ate: false, alive: false }
+    }
+  }
+  const next = [{ x: nx, y: ny }].concat(snake)
+  if (!ate) next.pop()
+  return { snake: next, ate, alive: true }
+}
+
+// Reject a 180° reversal; otherwise accept the new direction.
+function turn(current, desired) {
+  if (current.x + desired.x === 0 && current.y + desired.y === 0) return current
+  return desired
+}
+
+// Rotate a direction 90°: clockwise order right->down->left->up. Always ±90°, never a reversal.
+function rotateDir(dir, clockwise) {
+  const order = [{ x: 1, y: 0 }, { x: 0, y: 1 }, { x: -1, y: 0 }, { x: 0, y: -1 }]
+  let i = 0
+  for (let k = 0; k < 4; k++) {
+    if (order[k].x === dir.x && order[k].y === dir.y) { i = k; break }
+  }
+  return clockwise ? order[(i + 1) % 4] : order[(i + 3) % 4]
+}
+
+// Random empty cell, avoiding the snake.
+function spawnFood(snake, N) {
+  const occupied = new Set(snake.map(s => s.x + ',' + s.y))
+  if (occupied.size >= N * N) return null
+  let x, y
+  do {
+    x = Math.floor(Math.random() * N)
+    y = Math.floor(Math.random() * N)
+  } while (occupied.has(x + ',' + y))
+  return { x, y }
+}
+
+module.exports = { initialSnake, step, turn, rotateDir, spawnFood }