feat(js): m11 pong capstone (model, rules, view, app, page) and polling smoke test

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

js/solutions/pong/rules.js

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+// m11 pong rules: one pure `pongUpdate`, built from small steps that each return a new piece of state.
+import { H, W, PADDLE, PLAYER_X, CPU_X, WIN, initialPong } from './model.js'
+import { nextUint32 } from '../random.js'
+
+export const clamp = (lo, hi, x) => Math.min(hi, Math.max(lo, x))
+
+const clampPaddle = (y) => clamp(0, H - PADDLE, y)
+
+const movePlayer = (state) =>
+  clampPaddle(state.player + (state.held.down ? 1 : 0) - (state.held.up ? 1 : 0))
+
+// The cpu chases the middle of its paddle toward the ball, one cell every other frame.
+const moveCpu = (state, frame) =>
+  frame % 2 === 0 ? clampPaddle(state.cpu + Math.sign(state.ball.y - PADDLE / 2 - state.cpu)) : state.cpu
+
+const hits = (paddle, y) => paddle <= y && y < paddle + PADDLE
+
+// Where the ball goes next, given the paddles' new positions. `scorer` says who got the point, if anyone.
+const moveBall = (state, player, cpu) => {
+  const { ball, vel } = state
+  const wall = ball.y + vel.dy < 0 || ball.y + vel.dy >= H
+  const dy = wall ? -vel.dy : vel.dy
+  const ny = ball.y + dy
+  const nx = ball.x + vel.dx
+  const bounce =
+    (nx === PLAYER_X && vel.dx < 0 && hits(player, ny)) ||
+    (nx === CPU_X && vel.dx > 0 && hits(cpu, ny))
+  return {
+    ball: { x: bounce ? ball.x : nx, y: ny },
+    vel: { dx: bounce ? -vel.dx : vel.dx, dy },
+    scorer: nx < 0 ? 'cpu' : nx >= W ? 'player' : null,
+  }
+}
+
+// After a point the ball restarts in the middle, heading toward whoever conceded; the seed picks up or down.
+const serve = (state, scorer) => {
+  const [r, seed] = nextUint32(state.seed)
+  const score = { ...state.score, [scorer]: state.score[scorer] + 1 }
+  return {
+    ...state,
+    ball: { x: 40, y: 25 },
+    vel: { dx: scorer === 'cpu' ? -1 : 1, dy: r % 2 === 0 ? 1 : -1 },
+    score,
+    seed,
+    status: score[scorer] >= WIN ? 'Over' : 'Playing',
+  }
+}
+
+const frame = (state) => {
+  if (state.status !== 'Playing') return state
+  const n = state.frame + 1
+  const player = movePlayer(state)
+  const cpu = moveCpu(state, n)
+  const { scorer, ...moved } = moveBall(state, player, cpu)
+  const next = { ...state, ...moved, player, cpu, frame: n }
+  return scorer === null ? next : serve(next, scorer)
+}
+
+export const pongUpdate = (state, action) => {
+  switch (action.tag) {
+    case 'Press': return { ...state, held: { ...state.held, [action.key.toLowerCase()]: true } }
+    case 'Release': return { ...state, held: { ...state.held, [action.key.toLowerCase()]: false } }
+    case 'Restart': return initialPong(state.seed)
+    case 'Frame': return frame(state)
+    default: return state
+  }
+}