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. Milestone 3
adds a nesting-complexity heatmap to the lint output with a configurable
--max-nesting-depth threshold.

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

Milestone 3 detail

Added a nesting-complexity heatmap to the mint-ds lint output:

• lib/css-lint-rules.mjs — new nestingDepthHeatmap(css, opts) function that
  aggregates parseNestedRules() into a per-depth rule-count distribution
  (depth 0 through maxDepth) and flags every rule nested deeper than the
  threshold (default 3) in excessive.
• bin/mint-ds.mjs — cmdLint now renders the heatmap as an ASCII bar chart
  (green below the threshold, yellow at it, red above it) and lists a WARN line
  for each rule exceeding the threshold. New --max-nesting-depth <n> flag
  (documented under LINT OPTIONS in --help); invalid or missing values fall
  back to the default of 3.
• lib/__tests__/css-lint-rules.test.mjs — 5 new tests covering flat CSS,
  multi-level distribution, default/custom thresholds, and empty input.

How to test:

npm run typecheck
npm test
npx mint-ds lint <dir> --max-nesting-depth 2

[nujovich]

All milestones complete. Ready for review when you are.