feat(site): landing links, what's new, shared tokens, CI guardrails

[TMHSDigital]

Summary

PR 3 of 3 in the website QoL pass. Merge after #303 (the replacement for #301; #300 is already on main). Retargeted to main so it survives the #303 branch deletion; until #303 lands, this diff also shows #303's changes.

Landing (scripts/site/)

• Skill names link to their SKILL.md; each skill has a "When to use (N)" disclosure listing its ## Trigger bullets (already parsed, never rendered before). Rule names link to their .mdc.
• Skills and Rules panels each get a filter box (rendered hidden, revealed by JS; the count reads "N of M"; an empty-state line appears when nothing matches).
• What's new panel (between Showcase and Skills): the latest release + release-notes/changelog links, and the 6 most recently added examples/showcase pieces dated by the commit that first added each dir. Gallery JSON order is not add order (gn-sim-fountain is newest but not last). A shallow clone returns [] and the panel shows only the release line.
• Removed: six no-op collapse carets, two inline style= attributes. Blender versions come from a new site.json blender block instead of four hardcoded strings. HUD corners' opacity:0 is gated on html.js. The copy button has an execCommand fallback and a failure state. The showcase CTA uses gallery/#k=showcase.
• Mobile: long code tokens in skill descriptions no longer widen the 1fr column past the panel (0 overflowing rows at 375px).

Shared tokens

• scripts/site/tokens.css was a dead fleet leftover (GitHub palette, unused). It is now the single :root palette, included by template.html.j2 and 404.html.j2, and inlined by build_gallery.py (tokens_css()). The gallery's values are byte-identical to before (aliases --surface, --text-dim and --font-sans map onto the shared names). The 404 page's --panel moves from #232428 to the shared #222327.

CI

• New validate-site job: setup-python, pinned requirements.txt, build the landing page into docs/, then run tests/check_site_links.py.
• pages.yml: fetch-depth: 0, pip install -r scripts/site/requirements.txt (the pin was previously ignored), and the same link check before upload.
• tests/check_site_links.py (stdlib): walks docs/index.html, docs/404.html and docs/gallery/**/index.html. It resolves every relative or site-absolute href/src, checks that each #anchor exists on its target page, skips gallery filter-state fragments containing =, and requires alt on every <img>.

Evidence

Proven by live run:

• check_site_links.py: 91 pages, 2593 internal references, 0 failures.
• Falsification: in docs/gallery/bmesh-gear/index.html I changed ../driver-wave/ to ../driver-wav/, #run to #runx, and one alt= to data-alt=. Result: exit 1 with 3 failures (missing file, missing id runx, <img> without alt). Restored via regeneration: exit 0.
• recent_additions() returns gn-sim-fountain, book-trolley, bookshelf, butter-churn, wooden-yoke, apothecary-shelf, which matches the merge history (feat(examples): add gn-sim-fountain #299, feat(showcase): add book-trolley #284, feat(showcase): add bookshelf #283).
• Playwright at 1440px:
  • The skills filter "bmesh" gives "1 of 16"; "zzzz" shows the empty state; clearing restores "16 loaded".
  • The rules filter "export" leaves 2 rows. There are 16 trigger disclosures and 9 rule links.
  • HUD opacity is 1 after boot. This caught and fixed a specificity bug where html.js .hud-corner outranked .booted .hud-corner.
• Playwright at 375px: no horizontal overflow, 0 overflowing skill rows.
• The gallery build is deterministic after the tokens change (two runs, diff -rq clean).

By inspection only: the validate-site and pages.yml changes will first run in CI on this PR once it targets main. The shallow-clone fallback was not exercised live.

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