docs: restructure DESIGN.md into Geist-inspired foundations + docs/ui product rules

[YaelAnaya]

What changed

Documentation-only restructuring of the frontend design documentation. DESIGN.md is rewritten as a concise Geist visual-foundations and interaction contract (Purpose, Philosophy, Foundations, Component Principles, Responsive, Interaction, Accessibility, Content and Copy, Implementation Boundaries — 1453 → 437 lines). Every product rule it used to hold is relocated, not deleted:

• docs/ui/product-principles.md — data-first, density on data surfaces, action-forward screens, never-disable-without-explanation, blast-radius confirmations, the authoritative product status mapping, number formatting, list/section rules, tabs, the Home and project-view screen rules, component contracts, and the current layout specifics (page/dialog widths).
• docs/ui/navigation.md — ancestor chains and breadcrumbs, the rail's membership rule and widths, URL state ("not everything selectable is a place"), Back semantics, callbacks-not-routes.
• docs/ui/annotator.md — the whole annotation-workspace contract: top bar zones, side panel regions, tool strip, frame gallery, zoom and the device model, read-only mode, the stage, shape rendering, provenance and confidence rules.
• docs/architecture/frontend/ui-core.md — gains the Libraries section (technology constraints moved out of the visual contract).
• Cross-references repointed: CONTRIBUTING.md, docs/README.md, docs/ui.md, docs/annotations.md, architecture READMEs, two .agents/skills files; the three new pages registered in docs-site/src/sidebar.mjs.

Foundation changes at the documentation level: primitive-scale + semantic-alias color model (light/dark-ready) with stage kept as a VisionSet-specific token; brand separated from functional semantics (the exactly-two-sites/count rules removed); Geist Sans/Mono with semantic type roles; radius by material level (6/12/16/full); materials replacing generic shadow tiers; action hierarchy replacing the one-filled-button count (and the button-goes-green-for-save principle removed); a general async-feedback baseline replacing issue-specific timing history; Title Case for labels / sentence case for prose replacing sentence-case-everywhere; copy bans replaced by principles.

No production file changed: nothing under frontend/, src/, or tests/.

Deliberate tensions, reported

• The task spec asked to remove tests based on counting filled buttons / brand sites; it also declares the phase documentation-only with frontend/ui-core/src/screens/* out of scope. The tests stay (they enforce the current, unchanged implementation); removing them belongs to the token/primitive implementation phase. Until then, tests/scripts/design_tokens.test.mjs's brand-site roster and the screen-level button-count tests enforce rules DESIGN.md no longer states — they pin the implementation, not the contract.
• The parallel shadcn migration branches (feat/shadcn-foundation, feat/shadcn-primitives) describe the same target direction from the implementation side; sequencing them against this branch is a review decision.

Found, not fixed

• tests/scripts/favicon.test.mjs asserts hex #eb5a47 as "the Robomous accent" while the brand token is #e85d44 — pre-existing, out of scope here.
• docs/annotations.md's showcase section still describes v1-era demo styling (#d0d7de borders, "Robomous orange as accent"); it describes the demo page and predates this change.

Test plan

All checks ran inside Linux Docker (visionset-gate:latest, which clones the committed branch from the repo mounted read-only, so line endings materialize as in CI):

docker run --rm --ipc=host -v "C:/Users/yaela/dev/VisionSet:/src:ro" visionset-gate:latest \
  bash -c 'git clone -b docs/geist-design-foundations /src /work && cd /work && \
           pnpm install --frozen-lockfile && pnpm test:scripts && \
           pnpm --dir docs-site install --frozen-lockfile && bash scripts/check.sh docs'

• pnpm test:scripts → 107 pass / 0 fail (includes docs_links, docs_sidebar, design_tokens).
• bash scripts/check.sh docs → PASSED (site build, deterministic projection, 4260 internal links across 47 pages resolve).

Browser and python suites are untouched by this diff; CI runs the exhaustive matrix.

#706 and the shadcn branches are untouched.