feat(javascript): the Generational Collection story, in 2D and 3D

[mnaimfaizy]

The Generational Collection story, from the approved spec docs/stories/javascript/generational-collection.md, in 2D and 3D. It sits in the Garbage Collection section, after the Garbage Collection story.

What it shows

cache = { calls: 0 } and batch() run twice. Each run makes temporary objects that die young. A minor collection copies cache from semi-space A to semi-space B and updates the reference to point at the copy. All of A is then freed in one step, without visiting the dead objects (the aha, beat 6: a collection costs what survives, not what died). The spaces swap roles, and a second collection promotes cache to the old generation. 10 beats.

Changes

• Snapshot: four optional fields that traces never set: Snapshot.spaces, HeapObject.copyOf, a copied state and a scavenge phase, plus helpers (spaceOf, isSpaceFreed, originOf, getCopyTrails…). A test traces the story's program and checks none of them appear.
• 2D and 3D views: one heap region per space, with FROM-SPACE / TO-SPACE / FREE tags; COPIED objects at half strength; a freed space drawn once, faint, with no per-object RECLAIMED tag; a dotted copy trail, and a short glide in 3D (none with reduced motion). Fixture tests prove snapshots without spaces lay out exactly as before in both views.
• Fix to the shared 2D view: the enter animation held opacity at 1, so collected objects never faded. They now do, in the Garbage Collection story too.
• Section: the story, a pointer from the first story's note, a "Young generation" card, and "weak" dropped from "generational hypothesis". No Playground example: a run cannot observe a collection.
• Research report 6.3: one paragraph on what a scavenge does, checked against V8's "Trash talk" (source 26).
• Quiz: javascript-q34 to q36.
• beat-model skill: a moved object is drawn as a copy, never as an object changing cell.

Validation

• npx tsc -b, lint, npm run test:run (1,505 tests in 106 files), build, agents:check, and the lazy-chunk check all pass.
• Browser at 1280px: 2D beats 4, 6 and 7 (labels 11.3px); 3D beats 4 and 8. A fresh load stepped through all ten beats in 3D and back to 2D with no console errors. The Garbage Collection story's beat 8 still reads correctly with the new fade.
• Not checked in the browser: the no-WebGL fallback and Hold view.

Known

• The Garbage Collection story's 3D HEAP caption overlaps #draft at 800px wide on beats 1, 2, 7 and 9. This was already the case before this PR.

🤖 Generated with Claude Code