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/app.js

@@ -0,0 +1,31 @@
+// m11 pong app: the same wiring as app.js (streams in, drawings out), reusing the course's modules.
+import { initialPong, PongAction } from './model.js'
+import { pongUpdate } from './rules.js'
+import { pongView } from './view.js'
+import { map as mapIO, randomSeed, drawScene } from '../io.js'
+import { keyDowns, keyUps, interval, map, filterMap, merge, scan, startWith } from '../stream.js'
+
+export const FRAME_MS_PONG = 30
+
+const keys = {
+  ArrowUp: 'Up', w: 'Up',
+  ArrowDown: 'Down', s: 'Down',
+}
+
+// hasOwn, not `key in keys`: "constructor" and "toString" are inherited, not keys we mapped.
+export const pongKey = (isDown, key) =>
+  Object.hasOwn(keys, key) ? (isDown ? PongAction.Press : PongAction.Release)(keys[key])
+    : key === 'r' && isDown ? PongAction.Restart
+    : null
+
+export const pongActions = merge(
+  map(interval(FRAME_MS_PONG), () => PongAction.Frame),
+  merge(
+    filterMap(keyDowns, (key) => pongKey(true, key)),
+    filterMap(keyUps, (key) => pongKey(false, key))))
+
+export const pongStates = (seed, actions) =>
+  startWith(scan(actions, pongUpdate, initialPong(seed)), initialPong(seed))
+
+export const pongMain = mapIO(randomSeed, (seed) =>
+  map(pongStates(seed, pongActions), (state) => drawScene(pongView(state))))