Preserve class-owned column max-width geometry

[chubes4]

Summary

Keep class-owned max-width geometry in generated CSS instead of serializing unsupported core/column inline style.

What changed

• Removed max-width from core/column style serialization while retaining it in generated class geometry, with a regression contract proving parsed Gutenberg validity and preserved CSS.

How to test

1. Run cd php-transformer && php tests/contract/run.php; expect The transformer contract suite passes, including class-owned column max-width validity..
2. Run cd php-transformer && composer test:canonical; expect The canonical artifact compiler and HTML transformer suites pass..

Compatibility

The change narrows core/column serialization to attributes supported by Gutenberg save output; class-owned max-width remains rendered through the existing generated CSS path.

Evidence

• Base unchanged since verification: trunk remains at 0cd6571.
• Reviewer-resolvable evidence from source_refs[0]: Keep class-owned max-width off core/column serialized style #714
• Verified finalization base: trunk at 0cd6571
• canonical-suites: passed (165 artifact compiler and 355 HTML transformer assertions) (Passed)
• transformer-contract: passed (563 assertions) (Passed)

AI assistance

• AI assistance: Yes
• Tool(s): Homeboy (OpenCode via Homeboy)
• Model: openai/gpt-5.6-sol
• Used for: Diagnosed the invalid Gutenberg serialization, implemented the focused ownership fix and regression contract, and verified the canonical transformer suites; Chris Huber remains responsible for every line.

Source relationships

• Closes Keep class-owned max-width off core/column serialized style #714