chore(js): polish pong keys, held mapping and m09 main test

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

js/test/m11-pong.test.js

@@ -41,7 +41,9 @@ test('m11 clamp keeps a number inside [lo, hi]', () =>
 
 test('m11 pongUpdate does not modify the state it is given', () => {
   const s = freeze(initialPong(0))
+  const before = structuredClone(s)
   assert.doesNotThrow(() => pongUpdate(s, PongAction.Frame))
+  assert.deepStrictEqual(s, before)
 })
 
 // 3000 frames with no one touching the keys: the ball must always stay on the court and the match must end.