Extract author selector semantic preparation

[chubes4]

Summary

• extract author-style installation and selector semantic preparation from HtmlTransformer
• centralize selector matching, cache accounting, and control/inline/attribute/root/table marker discovery
• retain six explicit conversion-policy decisions in a typed context while removing 326 lines from the transformer

Part of #242.

Verification

• composer validate --strict
• composer test
• focused contracts: 100 author-selector assertions, 63 engine-support assertions, 22 compile-fragment assertions, shared-cache equivalence across 54 pages, session isolation, artifact projection, navigation projection isolation, and table conversion
• 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 map the semantic-preparation boundary, implement the extraction, and run the verification suite; the resulting diff and evidence were reviewed before publication.