feat(course): audit fixes and twelve extensions (Modules 7b, 12, 13)

Audit pass:
- Frame rule counts frames since the last move, so every step lasts
  exactly movesEvery(score) frames, also right after a speed-up
- clampTime + MAX_GAP_MS: a hidden tab no longer replays the whole gap
- Pong Restart keeps held keys
- Fact corrections (mulberry32 output coverage, Haskell IO, uniform)
- New fixture cases from a mutation analysis

Extensions (all three tracks, shared fixtures, course text):
- Turn queue (Exercise 10.6), persisted high score (10.7),
  replay viewer (10.8), play() end-to-end helper (9.5)
- Optional State (4.7) and optional Module 7b, stack-safe IO
- Module 12, property-based testing with generators, shrinking,
  invariants and a model-based oracle
- Module 13, Conway's Life with a Click event
- JS type checking via jsconfig, JSDoc and types.d.ts
- Maintainer tools: scripts/mutate.mjs, scripts/typecheck-js.sh,
  Playwright end-to-end suite (e2e/, scripts/e2e.sh)

Docs: docs/contract.md is the canonical contract; specs and build log
under docs/superpowers.

Verified: JS 728, Scala 689, F# 691 unit tests; stubs and progression
M01-M13 and M07b; 176 end-to-end tests on Chromium and WebKit.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

scala/web/Main.scala

@@ -1,9 +1,10 @@
 package snake
 package web
 
+import scala.scalajs.js
 import scala.scalajs.js.Dynamic.global
 
-// Two programs, one runtime: build-web.sh picks the entry point with --main-class.
+// Four programs, one runtime: build-web.sh picks the entry point with --main-class.
 
 private def canvas() = global.document.getElementById("game")
 
@@ -14,3 +15,29 @@ object SnakeMain:
 /** scala/pong.html: Pong (m11). */
 object PongMain:
   def main(args: Array[String]): Unit = Runtime.runApp(pong.PongApp.pongMain, BrowserPlatform(canvas()))
+
+/**
+ * scala/replay.html: the replay viewer (m10). The recording is plain JSON next to the test fixtures; the page is served
+ * from the repository root by scripts/serve.mjs, so it can be fetched (relative to the page, scala/replay.html).
+ * The JSON is decoded by hand with js.Dynamic: `{seed, actions}`, each action `{"tag":"Tick"}` or
+ * `{"tag":"Turn","dir":"Up"}`.
+ */
+object ReplayMain:
+  private def action(a: js.Dynamic): Action = a.tag.asInstanceOf[String] match
+    case "Tick" => Action.Tick
+    case "Turn" => Action.Turn(Direction.valueOf(a.dir.asInstanceOf[String]))
+    case other  => throw new IllegalArgumentException(s"a recording holds only Tick and Turn, not $other")
+
+  private def recording(json: js.Dynamic): Recording =
+    Recording(
+      json.seed.asInstanceOf[Double].toLong,
+      json.actions.asInstanceOf[js.Array[js.Dynamic]].toList.map(action)
+    )
+
+  def main(args: Array[String]): Unit =
+    val loaded = global.fetch("../fixtures/replay-sample.json").`then`((r: js.Dynamic) => r.json())
+    loaded.`then`((json: js.Dynamic) => Runtime.runApp(Replay.replayMain(recording(json)), BrowserPlatform(canvas())))
+
+/** scala/life.html: Conway's Life with the mouse (m13). */
+object LifeMain:
+  def main(args: Array[String]): Unit = Runtime.runApp(life.LifeApp.lifeMain, BrowserPlatform(canvas()))