feat(js): m09 input, app, runtime, browser, main, index.html and smoke test

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

js/solutions/runtime.js

@@ -0,0 +1,31 @@
+// The single impure place in the course (with browser.js and main.js): everything else only builds values.
+// Here we interpret IO descriptions against a Platform and hold the one mutable variable, the current stream.
+// Platform = { fillRect, fillText, randomSeed, startClock, onKeyDown, onKeyUp }
+
+// Perform one op on the platform and return its result.
+const perform = (op, platform) => {
+  if (op.tag === 'RandomSeed') return platform.randomSeed()
+  const s = op.shape
+  return s.tag === 'Rect'
+    ? platform.fillRect(s.x, s.y, s.w, s.h, s.color)
+    : platform.fillText(s.x, s.y, s.text, s.color, s.size, s.align)
+}
+
+// A loop, not recursion: a scene is hundreds of ops and the stack must not grow with them.
+export const runIO = (io, platform) => {
+  let current = io
+  while (current.tag === 'Effect') current = current.k(perform(current.op, platform))
+  return current.value
+}
+
+export const runApp = (main, platform) => {
+  let stream = runIO(main, platform)
+  const feed = (input) => {
+    const [ios, next] = stream.step(input)
+    stream = next
+    ios.forEach((io) => runIO(io, platform))
+  }
+  platform.onKeyDown((key) => feed({ tag: 'KeyDown', key }))
+  platform.onKeyUp((key) => feed({ tag: 'KeyUp', key }))
+  platform.startClock((t) => feed({ tag: 'Time', t }))
+}