Switch from sassc to jekyll-sass-embedded for faster builds

[rolfedh]

Summary

The site depends on the sassc gem (pulled in by jekyll-sass-converter 2.x). The sassc gem includes a native C++ extension that must be compiled from source during bundle install, which takes 1-2 minutes and is the single biggest bottleneck in local site setup and CI gem installation.

Suggestion

Switch to Dart Sass via the sass-embedded gem. The cleanest way is to bump jekyll-sass-converter to ~> 3.0: it dropped sassc in favor of sass-embedded in v3.0.0. (Note: there is no jekyll-sass-embedded gem.) Dart Sass ships prebuilt platform-specific binaries, so installation completes in seconds with no native compilation step.

Benefits

• Faster local setup: Eliminates the sassc native extension compilation (~1-2 min savings per fresh bundle install)
• Faster CI builds: Same benefit applies to CI pipelines that install gems from scratch
• Better Sass compatibility: Dart Sass is the primary Sass implementation and receives new features first. LibSass (which sassc wraps) has been deprecated since 2020.
• Simpler build dependencies: No C/C++ compiler toolchain needed for gem installation

Current state

The Gemfile already pins Jekyll 4.4.1 and an explicit converter:

gem "jekyll", "~> 4.4.1"
gem "jekyll-sass-converter", "~> 2.0"

Jekyll 4.4.1 already allows jekyll-sass-converter (>= 2.0, < 4.0), so no Jekyll bump is needed.

Switching involves:

1. Bumping the converter pin: gem "jekyll-sass-converter", "~> 3.0"
2. Regenerating Gemfile.lock and adding the CI platform: bundle lock --add-platform x86_64-linux (sass-embedded is a platform-specific gem; CI runs on ubuntu-latest)
3. Fixing one SCSS file that Dart Sass rejects (see Notes), then verifying the build

Notes

• LibSass has been deprecated since October 2020, so this is also a move toward a supported Sass compiler.
• Behavioral difference that requires a code fix: LibSass compiled imported .css files as Sass; Dart Sass treats .css as plain CSS. _sass/includes/brand.css uses Sass variables ($white, $black, $breakpoint-m), so Dart Sass fails with "Sass variables aren't allowed in plain CSS." Renaming it to _sass/includes/brand.scss fixes it (the @import "includes/brand" is unchanged).
• Dart Sass also logs deprecation warnings for @import and global color functions like darken(). These do not break the build (removal is slated for Dart Sass 3.0); migrating to @use/@forward and color.adjust() is a reasonable cleanup to bundle in.

[rolfedh]

Tested the migration — it works, with one required code fix

Confirmed the right path and built it against this repo (Ruby 3.2.3, the same jekyll build --future --config _config.yml,_only_latest_guides_config.yml CI uses):

• Gem change: bump jekyll-sass-converter to ~> 3.0 (it replaced sassc with sass-embedded in v3.0.0). There is no jekyll-sass-embedded gem — I've corrected the issue body. Jekyll is already 4.4.1, so no Jekyll bump is needed.
• sass-embedded installs as a prebuilt x86_64-linux-gnu binary — no C++ compilation, which is the speedup this issue is after. sassc is dropped from Gemfile.lock.
• Gemfile.lock needs the CI platform added (bundle lock --add-platform x86_64-linux), since sass-embedded is platform-specific.
• One required code fix: _sass/includes/brand.css uses Sass variables ($white, etc.). LibSass compiled imported .css as Sass; Dart Sass treats .css as plain CSS and errors with "Sass variables aren't allowed in plain CSS." Renaming it to _sass/includes/brand.scss resolves it (the @import "includes/brand" is unchanged).
• One color-function fix: the only deprecated color function in this repo's own SCSS was darken(#ee802f, 10%) in _sass/layouts/blog.scss, changed to color.adjust(#ee802f, $lightness: -10%) (with @use "sass:color").

Deprecation warnings that remain (non-blocking):

• @import is deprecated in Dart Sass (removal only in the unreleased Dart Sass 3.0). Migrating our ~64 @imports to @use/@forward is a larger, riskier change, so I'm leaving it for a dedicated follow-up PR rather than bundling it here.
• The bulk of the remaining warnings come from the minima theme gem (minima 2.5.1, theme: minima), not this repo — those are upstream's to fix.

Planned PR (Fixes #2551): Gemfile + Gemfile.lock, rename brand.css → brand.scss, and the blog.scss color-function fix. Build passes; sassc/native compilation is gone.