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>

js/solutions/model.js

@@ -4,12 +4,16 @@
 /** @typedef {'Up'|'Down'|'Left'|'Right'} Direction */
 /** @typedef {'Playing'|'Paused'|'GameOver'|'Won'} Status */
 /** @typedef {{width: number, height: number, snake: Point[], dir: Direction, nextDir: Direction,
- *   apple: Point|null, seed: number, score: number, status: Status, frame: number}} GameState */
+ *   apple: Point|null, seed: number, score: number, status: Status, frame: number,
+ *   queued: Direction|null}} GameState */
+/** @typedef {{tag: 'Tick'} | {tag: 'Turn', dir: Direction} | {tag: 'TogglePause'} | {tag: 'Restart'} | {tag: 'Frame'}} Action */
 
-/** A grid cell. */
+/** A grid cell.
+ * @type {(x: number, y: number) => Point} */
 export const point = (x, y) => ({ x, y })
 
-/** Structural equality: same x and same y. */
+/** Structural equality: same x and same y.
+ * @type {(a: Point, b: Point) => boolean} */
 export const pointEquals = (a, b) => a.x === b.x && a.y === b.y
 
 const deltas = {
@@ -19,15 +23,19 @@ const deltas = {
   Right: point(1, 0),
 }
 
-/** The unit step for a direction (y grows downward). */
+/** The unit step for a direction (y grows downward).
+ * @type {(dir: Direction) => Point} */
 export const delta = (dir) => deltas[dir]
 
+/** @type {Record<Direction, Direction>} */
 const opposites = { Up: 'Down', Down: 'Up', Left: 'Right', Right: 'Left' }
 
-/** The direction pointing the other way. */
+/** The direction pointing the other way.
+ * @type {(dir: Direction) => Direction} */
 export const opposite = (dir) => opposites[dir]
 
-/** Everything that can happen to the game, as data. `Turn` is a function because it carries a direction. */
+/** Everything that can happen to the game, as data. `Turn` is a function because it carries a direction.
+ * @type {{Tick: Action, Turn: (dir: Direction) => Action, TogglePause: Action, Restart: Action, Frame: Action}} */
 export const Action = {
   Tick: { tag: 'Tick' },
   Turn: (dir) => ({ tag: 'Turn', dir }),
@@ -36,7 +44,8 @@ export const Action = {
   Frame: { tag: 'Frame' },
 }
 
-/** A fresh 20x20 game: 3-segment snake heading Right, no apple yet. */
+/** A fresh 20x20 game: 3-segment snake heading Right, no apple yet, nothing queued (`queued` is used from m10 on).
+ * @type {(seed: number) => GameState} */
 export const initialState = (seed) => ({
   width: 20,
   height: 20,
@@ -48,4 +57,5 @@ export const initialState = (seed) => ({
   score: 0,
   status: 'Playing',
   frame: 0,
+  queued: null,
 })