Extract high-value style boundary policy

[chubes4]

Summary

• Extract high-value static CSS matching boundary recognition from StyleResolutionTrait into HighValueStyleBoundaryPolicy.
• Keep StyleResolutionTrait's existing private entrypoint as a delegate so behavior stays unchanged.
• Leave fixtures and CHANGELOG.md untouched.

Root cause / technical debt addressed

HtmlTransformer's style-resolution path still carried generic pattern-recognition policy inline in the trait. That made future additions to static CSS matching boundaries a pile-on conditional risk. This PR gives that boundary decision a dedicated policy class without changing the conversion contract.

Testing

• composer test (php-transformer)
• Includes canonical contracts, unit tests, parity fixtures (228 fixtures), and packaging proof.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Inspected transformer structure, extracted the style-boundary policy, ran verification, and drafted this PR description.