feat(docs): add a print stylesheet for documentation pages (DX-063)

[Ay-obami]

Closes #575
Closes #582
Closes #579

Summary

Adds a print stylesheet for the documentation site so reference pages (particularly /resources/terms and /concepts/risk) print as clean, light, content-only documents instead of a navigation header followed by theme-dependent, unreadable output.

• apps/docs/src/styles/print.css (new) — @media print rules that:
  • hide the page chrome: static page header, DocsLayout sidebar/TOC/header/footer (data-slot hooks), search dialog, mobile TOC, heading anchors, and the (not-yet-built) reading-progress / feedback controls via documented hooks;
  • force the light palette regardless of the active theme by re-declaring every token the .dark block in packages/ui/src/styles/globals.css overrides with that file's :root light values;
  • expand the content column to the full page width;
  • expand every collapsible (grid-rows override) and tab panel — inactive panels stay mounted and get data-tab-label prefixes so the reader can still tell the alternatives apart;
  • print the destination URL after internal links (main a[href^="/"]::after);
  • avoid page breaks inside code blocks, callouts, and table rows (break-inside: avoid + repeating thead);
  • render a print-only footer with the page URL and last-updated date.
• The rules live in a docs-print cascade layer declared before Tailwind's theme, base, components, utilities layers, so their !important rules beat the base-layer preflight [hidden] { display: none !important } that guards inactive tab panels (Base UI keepMounted panels and the content Tabs both hide inactive panels with the hidden attribute).
• scripts/build.ts renders the print footer (canonical URL from the new DEFAULT_SITE_URL in src/lib/seo.ts + the frontmatter updated date) — no client JS is needed for any print path, including headless PDF export.
• mdx/components.tsx content Tabs now keep every panel mounted (inactive ones hidden via the hidden attribute), so print/search can reach content that was previously never rendered to the DOM; this matches the primitives-based Tabs (keepMounted) convention. mdx/Tabs.tsx panels carry data-tab-label.
• DocsLayout.tsx marks its chrome regions with data-slot hooks (docs-shell/header/sidebar/toc/footer) that the print sheet targets.

Pre-existing main defects repaired (required to build/verify this change)

• scripts/build.ts — the feat(docs): implement DX-027, DX-030, DX-033, and DX-037 #636 merge left an orphaned inline-styled page template referencing undefined variables, and removed the vite stylesheet build that the page template's /assets/${stylesheet} link depends on. Restored the bunx vite build + hashed-stylesheet pipeline; bun run --cwd apps/docs build now completes (25 static routes).
• scripts/check-content.ts — the glossary check crashed on entry.answer (a field headingEntries() no longer returns); it now reads each entry's own section of the glossary body. The home page is reachable by definition, so it is exempted from the orphan-page check.
• scripts/generate-faq.ts — same stale entry.answer crash; each FAQ entry's answer is now derived from its own section body.
• mdx/components.tsx — removed the import { Tab, Tabs } from "./Tabs" that conflicted with the local Tabs declaration (TS2440).

Verification

• bun run --cwd apps/docs build — all content gates pass, 25 static routes + Pagefind-ready output.
• bun test (apps/docs) — 54 pass / 12 fail / 3 errors; the failures are identical on clean main (pre-existing: Sidebar/Pager route fixtures, docs shortcuts, test-hook timeouts) plus a new passing test MDX content tabs keep every panel mounted for print and search.
• Headless-Chrome (Playwright) PDF exports of three representative pages with DOM assertions under print emulation:
  • /resources/terms — terms.pdf
  • /concepts/risk — risk.pdf
  • /resources/faq — faq.pdf
  • Each verified: header hidden, print footer present with Last updated <date> · https://docs.so4.market/<route>, internal links print their destination (… (/concepts/risk)), and dark-theme variants (terms-dark-active.pdf, risk-dark-active.pdf) print identically in the light palette when .dark is forced on <html>.
  • A fixture harness against the built stylesheet confirms hidden tab panels expand, collapsibles expand, and DocsLayout chrome slots hide in print.
• PDFs and the reproduction scripts are in the local print-exports/ folder — attach the three PDFs to this PR (GitHub doesn't accept file uploads via the API).

Notes

• Dependency (DX-062, issue DX-062: Add reading time and reading progress #574) has not landed — the reading-progress indicator does not exist on main yet. The print sheet hides the header (where DX-062's indicator is specified to live) and documents data-reading-progress / data-page-feedback / print:hidden as the convention for components that land later.
• /resources/terms is status: draft, so the real build intentionally excludes it; its PDF was rendered locally with the exact production template (print-exports/render-draft-terms.ts) for verification only.
• The current static renderer emits headings/lists/callouts but no <pre>/<table>/tabs yet (the full MDX pipeline is deferred per the DX-097–100 note in docs/dx_1/001_docs_site.md); the print rules for those elements are in place and verified via the fixture harness so they apply the moment the renderer catches up.
• Light-mode --surface-*/--text-* and status subtle token values are not declared in :root upstream (light mode currently degrades via inheritance, and the subtle fills are dark tints that print as near-black boxes); print states the surface/text values explicitly and neutralizes callout fills to bordered boxes so printed callouts are readable.

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