docs(course): fix review findings in modules 00–05

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

course/02-the-game-as-data.md

@@ -26,7 +26,7 @@ After this module you can:
 
 ### Design it yourself first
 
-Take a sheet of paper and fifteen minutes. **Do not read past the "Stop here" line until you have written something down.** It does not matter if your answer differs from the course's; what matters is that you have thought about the questions, so that the canonical model answers questions you have actually asked.
+Take a sheet of paper and fifteen minutes. **Do not read past the "Stop here" line until you have written something down, and do not open this module's stub file yet** (`js/src/model.js`, `scala/src/Model.scala`, `fsharp/src/Model.fs`): its type declarations already contain the course's answer. You will open it for the exercises, after the reveal. It does not matter if your answer differs from the course's; what matters is that you have thought about the questions, so that the canonical model answers questions you have actually asked.
 
 Your task: write down the type of "the state of a game of Snake", in the language of your track or in pseudo-code. Use these questions to guide you:
 
@@ -47,9 +47,9 @@ Your task: write down the type of "the state of a game of Snake", in the languag
 
 ### The canonical model
 
-Here is the model the course uses. All three tracks use the same fields with the same meaning, which is what lets one JSON fixture test all three.
+Here is the model the course uses. (Now you may open your stub file: in Scala and F# these declarations are already written there for you, and in JS they are the JSDoc comments at the top of `model.js`.) All three tracks use the same fields with the same meaning, which is what lets one JSON fixture test all three.
 
-**JavaScript** (JS has no types, so the shape is documented with JSDoc in `model.js`)
+**JavaScript** (JS has no types, so the shape is documented with JSDoc comments)
 ```js
 /** @typedef {{x: number, y: number}} Point */
 /** @typedef {'Up'|'Down'|'Left'|'Right'} Direction */
@@ -59,7 +59,7 @@ Here is the model the course uses. All three tracks use the same fields with the
 // Action: {tag:'Tick'} | {tag:'Turn', dir} | {tag:'TogglePause'} | {tag:'Restart'} | {tag:'Frame'}
 ```
 
-**Scala** (already in `Model.scala`)
+**Scala**
 ```scala
 final case class Point(x: Int, y: Int)
 enum Direction:
@@ -403,6 +403,20 @@ Replace `get Tick() { return todo('m02') }` by `Tick: { tag: 'Tick' }`, and so o
 
 </details>
 
+<details><summary>Exercise 2.4 (Action) — near-solution (try the others first)</summary>
+
+```js
+export const Action = {
+  Tick: { tag: ___ },
+  Turn: (dir) => ({ tag: ___, ___ }),
+  TogglePause: ___,
+  Restart: ___,
+  Frame: ___,
+}
+```
+
+</details>
+
 <details><summary>Exercise 2.5 (initialState) — nudge</summary>
 
 This is a single record literal. Read the fixture `m02-initial-state.json`: the `expected` value is exactly the record you must build (with the seed from `args`).