Implement catwalk config differ

[williamthorsen]

Parent: #173 — Milestone 2

Goal

Create a structural differ that compares old and new CatwalkSceneConfig to produce change descriptors, enabling animated transitions instead of full-scene teardown.

Tasks

• Create src/client/visualizations/catwalk/state/catwalk-differ.ts
• Detect changes:
  • Orchestrator position changed (stationIndex diff) → triggers walk
  • Orchestrator working state changed → triggers glow toggle
  • Agent state changed → triggers visual state transition
  • Gate opened → triggers shrink animation
  • Artifact added to station → triggers appearance
  • Agents added/removed (rare, but possible if phase configuration changes)

Acceptance criteria

• Differ is a pure function: diffCatwalkConfig(prev, next) → CatwalkDiff
• Unit tests cover: no change, orchestrator moved, agent state changed, gate opened, artifact added

[williamthorsen]

Insight from implementation run (2026-03-06)

Composite identity keys in structural differs need all discriminant fields. The artifactKey function initially built its key from stationIndex:label:version, omitting the slot field. Three independent reviewers (core, test, code) all flagged this — the convergence validates the parallel review approach for catching subtle identity bugs that any single reviewer might rationalize away as "currently unused."

The fix was straightforward (add slot to the key), but the bug would have been invisible until input-slot artifacts were introduced, at which point it would have silently dropped additions.