Add shared artifact color constants and catwalk dimension/timing constants

[williamthorsen]

Goal

Establish the constants files at the correct levels so that actors and layout code can reference them.

Tasks

Shared constants (src/shared/constants/)

• Create artifact-colors.ts mapping artifact types to colors:
  • reqs, xplan, arch, plan, code, review, fixes, clean, holi, summary
  • Use the pastel tones from the prototype (e.g., reqs → #dee2e6, code → #fff3bf)

Catwalk constants (src/client/visualizations/catwalk/constants/)

• Create dimensions.ts:

  • Canvas size (CANVAS_W = 1400, CANVAS_H = 500)
  • Vertical positions (CATWALK_Y = 100, GROUND_Y = 340, CHUTE_TOP, CHUTE_BOT)
  • Entity sizing (agent radius, orchestrator radius, artifact width/height, gate width)
  • Layout margins

• Create timing.ts:

  • Walk speed, chute duration, work duration, pause duration

Acceptance criteria

• Constants are importable from their respective paths
• No duplication of color values between shared and catwalk constants
• Existing palette.ts and role-types.ts are not modified

[williamthorsen]

Insight from branch 177 implementation (run 20260305-101436Z):

CANVAS_H was set to 540 instead of the ticket's stated 500. The prototype on origin/162 uses 540, and the downstream layout engine plan (#178) explicitly depends on this value. The implementation followed the prototype as the canonical source. The ticket description may need updating to reflect 540 as the correct value.