feat(mint): add unused CSS custom properties detection

[nujovich]

Card: https://github.com/nujovich/mint-radar/issues/22

What: Adds deterministic dead-code analysis for CSS custom properties to Mint. Milestone 1 introduces the UnusedCustomPropertyIssue and UnusedCustomPropertiesAudit types in lib/types.ts, which track every custom property declared (--*) versus every property referenced (var(--*)), and adds an optional unusedCustomProperties field to AuditReport. Later milestones add the parser, CLI wiring with an optional --remove-unused flag, and Tailwind-scale test fixtures.

Why: Tailwind and other utility-heavy projects emit --tw-* and similar custom properties that are never consumed, inflating CSS output and complicating maintenance. No reliable automated tool detects truly-unused custom properties across CSS, HTML, and JS (PurgeCSS and Chrome Coverage both mishandle var() references and dynamically-injected properties). Mint can fill this gap with cross-file reference analysis, reinforcing its positioning as a thorough CSS health auditor.

Milestones

• Milestone 1 — Add UnusedCustomPropertyIssue and UnusedCustomPropertiesAudit types to lib/types.ts tracking defined (--) vs referenced (var(--)) custom properties, plus unusedCustomProperties field on AuditReport
• Milestone 2 -- Implement lintUnusedCustomProperties() in lib/css-lint-rules.mjs: walk all --* declarations and var() references, emit the defined-but-unused diff with selectors and cleanup suggestions
• Milestone 3 — Wire the unused-properties report into bin/mint-ds.mjs CLI output with a dead-variable list and an optional --remove-unused flag
• Milestone 4 — Add a test fixture with a large Tailwind-style stylesheet (1000+ custom properties) and tests in lib/tests/

How to test:

npm run typecheck
npm test

Milestone 4 detail

Added a Tailwind-style test fixture in lib/__fixtures__/tailwind-custom-properties.css: 1000 custom properties across 20 color palettes (50 shades each), with 750 referenced in utility classes and 250 declared but never used, simulating dead tokens left behind after a palette refactor.

Tests added to lib/__tests__/css-lint-rules.test.mjs (new lintUnusedCustomProperties at scale (Tailwind-style fixture) describe block, 4 tests):

• Detects all 1000 custom properties declared in the fixture
• Flags exactly 250 dead tokens left behind by the refactor
• Separates referenced palettes (red-fuchsia) from the unreferenced tail (pink-zinc)
• Attributes every dead token to :root with a cleanup suggestion

[nujovich]

All milestones complete. Ready for review when you are.