fix(compiler): preserve CSS docs across watch rebuilds

[Elements2720]

Summary

• preserve CSS custom-property metadata in Rollup transform cache entries
• replay cached style docs when TypeScript-only watch rebuilds recreate component metadata
• invalidate cached CSS transforms when a flattened stylesheet dependency changes
• cover docs-json style metadata, generated README CSS documentation, and dependency invalidation

Fixes #6939

Tests

• focused compiler/docs suites: 3 suites, 39 tests passed
• npm run ts scripts/index.ts -- --prod --ci passed
• targeted Prettier and ESLint checks passed
• git diff --check passed

The full Jest suite has one unrelated existing failure in src/compiler/output-targets/dist-lazy/test/generate-lazy-module.spec.ts; repository-wide type checking has existing Jest 28/29 dependency mismatch errors outside this change.