Clear three of four Sass deprecation categories

[amjjbonvin]

Addresses item 4 of the upgrade backlog.

Item 3 restored the real sass-embedded (dart-sass) toolchain in place of the
sassc (libsass) fallback, which surfaced deprecation warnings in four
categories. This clears three.

Category	Before	After
color-functions	present	cleared
global-builtin	present	cleared
slash-div	present	cleared
import	present	remains (see below)

Changes

• lighten() / darken() → color.adjust() with explicit percentages
• bare / division → math.div()
• global built-ins namespaced: round() → math.round(), unquote() → string.unquote()
• minima theme removed, config and gem

Verification

Every step was verified by rebuilding and byte-comparing the compiled
assets/css/main.css against a baseline captured before any change.

• main.css is byte-identical at every step
• 0 HTML pages changed
• The only site-wide output difference is the removal of three unused minima
  artifacts: assets/main.css, assets/main.css.map, assets/minima-social-icons.svg

Why minima was removed rather than patched

It contributed nothing the site uses: all six layouts in _layouts/ are
standalone with no parent layout, the site has its own _includes, and the only
stylesheet it produced (assets/main.css) is referenced nowhere. Removing it
cleared roughly 40% of the warnings on its own. Dropping the gem also removed its
unused jekyll-seo-tag dependency — there is no {% seo %} tag anywhere.

A bug found and deliberately NOT fixed

_sass/forms.scss has three declarations of the form margin-bottom: 5px / 2.
Sass never evaluated these: with two literals and no parentheses it emits the
slash verbatim, so the published CSS contains margin-bottom:5px/2 — invalid,
and silently discarded by browsers.

Converting them to math.div() would newly apply a 2.5px margin that has never
been in effect. That is a styling decision, not a deprecation fix, so they are
left as-is. They emit no warning. Flagged for a human call.

Why @import is deferred

It would touch ~25 files: assets/css/main.scss, twelve partials that rely on
variables.scss / mixins.scss being in global scope, and 13 @import
statements inside the vendored Font Awesome and Magnific Popup. Migrating only
our own files would not clear the category, since the vendored imports would
still warn. @import works until Dart Sass 3.0, which has no release date, and
migrating a frozen Font Awesome 4.x is throwaway work. The natural trigger is
whenever those vendored libraries are replaced.

Vendored files patched here each carry a comment noting the patch must be
re-applied if the library is updated.

Separately noted

_includes/_head.html loads assets/css/main_pretty.css — a committed,
optimised artifact, not the main.css compiled from _sass/. The two have
drifted: 1120 selectors in the compiled CSS vs 1118 in the shipped one, with 5
present only in the SCSS (never regenerated, so not live) and 3 only in the
shipped file. Nothing in the repo regenerates it; the Gruntfile has no CSS task.
Recorded as its own backlog item.

🤖 Generated with Claude Code

[amjjbonvin]

Copilot review:

Merge Readiness and Risk Assessment

Overall status: low risk and in good shape for merge
Why I’m comfortable:
the fix is narrow and mechanical
the generated CSS was checked for byte equivalence
no HTML output changed
the remaining @import migration is clearly deferred and documented rather than silently left out
No blocking issues found in the code changes themselves