Keep v2 SCSS byte-identical; add dart-sass…

Details

Installing a real compiler (dart-sass) and diffing the compiled CSS of
the pre-split tree vs this branch revealed that the iteration-2
hero-scroll merge reordered v2's compiled rules (behavior-preserving,
but not byte-identical). Revert that merge so all rules keep their
original source order:
- Restore _sass/v2/_hero-scroll.scss as a separate partial in its
  original manifest position (between db-loading and stats); _hero.scss
  goes back to hero rules only.
- Revert the _records.scss section comment to its original text.
Compiled CSS is now byte-identical to the original for every entry in
both compressed and expanded modes:
  style.scss  -> IDENTICAL (13850 B compressed / 17297 B expanded)
  v2.scss     -> IDENTICAL (19679 B compressed / 27635 B expanded)
Add reproducible tooling so CSS can be verified without a full Jekyll
build:
- sass (dart-sass) as a devDependency.
- frontend/scripts/check_scss.sh + `npm run css:check`, which compiles
  every front-matter stylesheet entry with load path _sass (mirroring
  jekyll-sass-converter) and fails on any compile error.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018Z78wTEczDqpD4nqJWKRME

Files changed (7)