feat(viz): shared stack-and-heap snapshot and a light/dark palette

[mnaimfaizy]

Summary

PR 3 of the ADR 0004 build plan: the shared foundation. No behaviour or visual change.

• src/shared/snapshot/ — Slot, StackFrame, HeapObject, Snapshot, Reference, slotKey, getReferences, getReachable, moved out of the Stack ↔ Heap story so a Playground trace step can hold the same shape.
  • Adds the heap kinds function and scope that a trace needs (a closure and the popped frame it keeps alive). No story uses them yet.
  • cell is optional, because a trace has no authored cells; Placed<T> marks authored placement.
  • Reachability stays derived from references, never stored.
  • The story narrows the generic types and re-exports every name it exported, so no other file changes. stackHeapStory.test.ts passes unedited and the beat data is untouched.
• src/shared/viz/palette.ts — one palette organised by role, with a light and a dark variant. Light reproduces today's colours exactly. Dark is defined and contrast-tested for the Playground and is not used yet. New roles: function and scope.
• StackHeap2D / StackHeap3D read their colours from the light palette; no raw hex remains in either file.
• beat-model skill — stack-and-heap stories take the snapshot from src/shared/snapshot; other domains (Git commits and refs) keep their own typed snapshots.

Checks

• npx tsc -b, npm run lint, npm run test:run (35 files, 364 tests; 24 new) and npm run build pass. npm run agents:check passes.
• Every hex removed from the two views exists with the same value in the light palette.
• The built entry bundle contains no WebGLRenderer; StackHeap3D is still its own lazy chunk.
• In a browser, the Stack ↔ Heap story at beat 7 renders in 2D and 3D with the original colours and no console errors. The other beats and the label-overlap script were not re-run.

Notes for review

• Light contrast. Three of today's light colours are below 3:1 against the light background: object stroke 2.42, array stroke 2.65, focus 2.05. The values are kept, and the test asserts the measured value for light only. Fixing them would change the published look.
• Behaviour edge. getReachable now ignores a refId that matches no heap object, and a slot may reference a frame id. No story has either case.
• Unused for now. frameActive fill and stroke, and the text / label entries (hex approximations of the current Tailwind classes) are used when the views are generalised in PR 4.
• The Git views keep their own palettes until then.

🤖 Generated with Claude Code