[Web][W1] Widget System Foundation (Design Tokens + Core Primitives)

Build the base design/token system and primitive RN-web components.

Acceptance Criteria

• Token model defined (color, type, spacing, radius, elevation, motion, theme).
• Core primitives implemented and documented.
• Accessibility and keyboard/focus behavior defined for primitives.
• SSR/hydration stability validated for primitive set.

Tembo-sized implementation issues

• #76 — visual constitution, tokens, themes, component gallery, and screenshot harness
• #77 — non-interactive layout, type, surface, and feedback primitives
• #78 — accessible interactive controls and overlays

Dependency order: #76 → (#77 and #78 in parallel). This parent is a visual design-system acceptance rollup and must not be assigned directly to an implementation agent.

automation:
  eligible: false
  executor_profile: tembo
  readiness: rollup
  primary_repository: TinyChain-Inc/client
  base_branch: main
  writable_scope: []
  prerequisites:
    - "#76"
    - "#77"
    - "#78"
  concurrency_group: ui-foundation-rollup
  required_commands: []
  external_actions: none
  draft_pr_only: true
  completion_authority: human
  visual_validation:
    required: true
    baseline_update_requires: human-visual-approval

This is a parent rollup, not an implementation entry point, and it names no files or tests. Start with #76, then review #77 and #78 in the stated dependency order; the parent is done only when those child issues satisfy the acceptance criteria and receive the required human visual validation.