feat(ui): add @pretable/ui shared design system package

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Summary

Introduces a new workspace package @pretable/ui containing the shared design system for the pretable playground, bench, and upcoming docs apps. No app migration in this PR — playground (Plan B), bench (Plan C), and the new docs app (Plan D) will consume this package in follow-up PRs.

What ships:

• Design tokens (tokens.css) — editorial (cream/ink/amber) + terminal (dark grid) + severity + syntax palettes, typography stacks, type scale, layout tokens. All --pt-* prefixed.
• Fraunces font via @fontsource-variable/fraunces (runtime dep, not self-hosted).
• Five presentational React components:
  • <Receipt> — inline numeric tag chip for body copy
  • <Callout> — cream-hi panel with amber/warn left border, role="note"
  • <CodeBlock> — terminal-surface snippet with optional tabs and copy-to-clipboard
  • <Nav> — 60px sticky header with wordmark, version pill, links, search trigger, GitHub star pill, CTA
  • <Footer> — one-line monospace footer with CI status indicator

Design + plan

• Spec: docs/superpowers/specs/2026-04-21-pretable-visual-system-design.md
• Plan: docs/superpowers/plans/2026-04-21-pretable-ui-package.md

Follow-ups folded into this PR

Two small improvements surfaced during review were committed onto this branch since they touch files that only exist here (couldn't be split into separate PRs without stacking):

• db3fdec chore(ui): tokenize Callout warn tag color — replaced the one hardcoded #7d4f0a with --pt-amber-ink-dark for full token coverage
• 828414a feat(ui): add semantic role to Callout — role="note" on both variants (with rationale in the commit about why role="alert" is wrong for persistent docs callouts)
• 10907d4 fix(ui): prevent Footer CI status from being announced twice — the dot's aria-label duplicated the adjacent visible "ci: {status}" text; now aria-hidden

Test plan

• 21 unit tests passing (pnpm --filter @pretable/ui test)
• Typecheck clean (pnpm --filter @pretable/ui typecheck)
• Lint clean (pnpm --filter @pretable/ui lint)
• Build emits dist/{index.js,index.d.ts,tokens.css,components.css}
• Full repo pnpm test green (no regressions in playground/bench/grid-core/etc.)
• Full repo pnpm typecheck clean

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Final review pass

Final holistic code review ran over the full package. Landed four in-scope polish items in 64afbbc:

• Tokenized the last two hardcoded hex values (--pt-grid-head, --pt-ink-hover) so components.css contains zero hex literals.
• @fontsource-variable/fraunces moved to optional peer dependency — package no longer pulls it unconditionally, matching the README guidance.
• Added "sideEffects": ["**/*.css"] and ./package.json to the exports map (tree-shaking + publint hygiene).
• Footer link key changed from label to href to avoid collisions.

Deferred (out of scope for this PR)

• Nav's LINKS array is module-level / non-customizable → worth a follow-up when a second consumer needs different links.
• CodeBlock copy button doesn't announce the "copied" state to screen readers → minor a11y refinement.
• tsup.config.ts uses cp in onSuccess, not cross-platform → only matters if we ever support Windows development.
• formatStars truncates at ≥1M → cosmetic.

CI status

test and format are failing on this branch. Both pre-exist on main — PR #6 was merged with the same two checks red. My branch only touches packages/ui/**; neither failure references those files:

• format: apps/bench/src/query-state.ts (not touched here).
• test: @pretable-internal/layout-core entry-resolution error from packages/grid-core (not touched here).

The three checks my package is responsible for are green: build, lint, typecheck.