fix(php-transformer): scan responsive variant styles

[chubes4]

Summary

• route responsive-document variant style extraction and CSS rebasing through StyleTagScanner
• retain styles larger than PCRE backtracking limits and cover the browser-visible breakpoint behavior

Closes #1266

Testing

• composer test in php-transformer (303 parity fixtures)
• npm test in php-transformer/tools/visual-parity (including the 390px browser regression)
• WordPress integration suite was not run locally; this change does not alter that suite and CI will verify it.

AI assistance

GPT-6 Astra via OpenCode was used for orchestration and review. openai/gpt-5.6-terra via direct OpenCode implementation was used to inspect, implement, test, and publish this change.