feat(mint): add CSS nesting depth audit

[nujovich]

Card: https://github.com/nujovich/mint-radar/issues/23

What: Adds a CSS nesting audit that flags nesting depth and specificity
anti-patterns. Milestone 1 added the NestingAudit type to lib/types.ts
with maxDepth, specificityGrowth, and unnecessaryAmpersand fields, plus
a nesting field on AuditReport. Milestone 2 adds parseNestedRules and
lintNesting to lib/css-lint-rules.mjs, which walk nested selectors,
measure maximum nesting depth, and flag redundant & prefixes. Remaining
milestones add the complexity heatmap output and fixture tests.

Why: CSS nesting is widely available since 2024, but recent write-ups
(Piccalilli, Kilian Valkhof) warn about the same pitfalls teams hit with Sass
nesting: excessive depth, uncontrolled specificity growth, and redundant &
selectors. Stylelint has basic nesting rules but no dedicated auditor that
measures maximum depth or detects unnecessary specificity growth. This closes
that gap.

Decision: PLAN defined 4 concrete code milestones.

Milestones

• Milestone 1 -- Add NestingAudit type to lib/types.ts with maxDepth,
  specificityGrowth, and unnecessaryAmpersand fields, plus a nesting
  field on AuditReport
• Milestone 2 -- Implement a parser that walks nested selectors with &,
  measuring maximum depth and flagging unnecessary & (when the nested
  selector is already specific)
• Milestone 3 -- Add a nesting complexity heatmap to the audit output with
  a configurable --max-nesting-depth threshold
• Milestone 4 -- Add test fixtures and tests based on Piccalilli and Kilian
  Valkhof nesting pitfalls

How to test:

npm run typecheck
npm test