fix: declare italic ClarityCity faces and test the custom.css<->static/fonts contract

[mrbobbytables]

Summary

Closes #317.

Two changes, matching the issue's recommendation:

1. src/css/custom.css — adds 9 @font-face blocks with
   font-style: italic for the ClarityCity 100-900 weights, pointing at
   the *Italic.woff2/woff/ttf files that were already committed under
   static/fonts/ but never referenced by any @font-face. Without
   this, every <em>/*italic* on the site rendered as a browser
   synthetic oblique instead of the real italic face, and 764K of font
   files shipped on every deploy as unreachable bytes. This is option
   (a) from the issue — restoring the intended typography at no
   download cost for readers who never hit italic text — over deleting
   the files, since they were clearly committed deliberately alongside
   their upright counterparts.
2. tests/css-assets.test.mjs — a new test-only file that reads
   every url() reference in src/css/custom.css and asserts:
   • the extraction finds a non-zero number of references (a non-vacuity
     guard: a regression in the regex cannot silently make the other
     assertions pass trivially)
   • each reference resolves to a real file under static/, compared
     case-sensitively per path segment via readdirSync (a case-only
     typo resolves on a case-insensitive developer filesystem and 404s on
     GitHub Pages)
   • every file under static/fonts/ is referenced by at least one
     @font-face in custom.css, so an orphaned font file fails the
     build instead of shipping silently

This lands as one PR rather than split in two, per the issue's own
note: the orphan-detection test is red against main until the italic
@font-face blocks exist, so the CSS change and the test that pins it
have to move together.

Verification

• npm run test:unit — 348 tests, 344 pass. The 2 failures are in
  tests/validate-architectures.test.mjs and are pre-existing on
  main at this branch's base commit (reproduced in isolation with
  node --test tests/validate-architectures.test.mjs before this PR's
  changes were applied); unrelated to custom.css/static/fonts/.
• npx prettier --check tests/css-assets.test.mjs src/css/custom.css — clean.
• npm run build — succeeds; build/fonts/ contains all 54 files, and
  the only warnings emitted are pre-existing HTML-minifier diagnostics
  on /, /architectures/, /resources/ unrelated to fonts or CSS.

Checklist

• Content speaks to end users (see CONTRIBUTING.md) — test-only/CSS change, no content
• Commits are DCO-signed (git commit -s)
• Site builds without new warnings

— hive: backend=copilot model=claude-sonnet-5

🐝 Hive Agent: contributor | SHA: f4335e8

[castrojo]

Reviewed: diff cleanly declares 9 italic ClarityCity faces (weights 100-900) in src/css/custom.css matching existing font files under static/fonts/, and adds unit tests in tests/css-assets.test.mjs that verify URL resolution.

Note on CI failure: The failure in 'Validate repository' is pre-existing on main due to tests/validate-architectures.test.mjs missing sourceUrl in its validRecord fixture after commit dc5fe09, unrelated to this PR's CSS/font changes.

Awaiting second/maintainer review and CI fix on main.