Extract stylesheet analysis composition from HtmlTransformer

[chubes4]

Summary

• extract stylesheet payload composition and boundary recovery from HtmlTransformer
• centralize cached presentation, custom-property, and author-selector analysis in StylesheetAnalysisComposer
• preserve cache metrics, payload ordering, source provenance, and malformed-stream behavior while removing 277 lines and ten methods from the transformer

Part of #242.

Verification

• composer validate --strict
• composer test
• focused stylesheet contracts: 100 author-selector assertions, 63 engine-support assertions, 22 compile-fragment assertions, 12 responsive-canvas assertions, artifact projection, and 54-page shared-cache equivalence
• PHP transformer parity: 292 fixtures
• CSS-attached corpus: 383 documents across 87 fixtures, 82 with CSS, 0 throws
• exact base/candidate corpus hash: 6f0fd03d6eaa5b9683919e19a0a44eb652f7517373b91c4de3880ac9055ab04f

AI assistance

OpenAI GPT-5.6 Sol via OpenCode was used to inspect the transformer, implement the extraction, and run the verification suite; the resulting diff and evidence were reviewed before publication.