Fix paragraph save-shape regressions

[chubes4]

Fixes #604.

Summary

• Remove unsupported style.dimensions.maxWidth from core/paragraph comment attributes and rendered markup.
• Preserve paragraph max-width through the existing deterministic be-inline-geometry-* carrier stylesheet.
• Add fixture-derived serialized-block regressions for 10-nonprofit, 13-realistic-small-business, and 74-lumen-coffee.

Why

Gutenberg paragraph save() does not reproduce dimensions.maxWidth. Blocks Engine emitted both the geometry carrier and inline max-width, so editor validation compared core's save output against extra markup and marked the paragraph invalid. The carrier already preserves the visual width; removing the unsupported duplicate representation restores save-shape parity.

The fixture regressions also prove generated color support classes stay out of className and has-text-color appears exactly once in rendered paragraph markup.

How to test

1. Check out this branch on a fresh clone.
2. Run cd php-transformer.
3. Run composer install.
4. Run composer test.
5. Confirm the contract suite exercises 10-nonprofit, 13-realistic-small-business, and 74-lumen-coffee, with all 230 parity fixtures and the packaging proof passing.
6. Run git diff --check from the repository root.

Backward compatibility

This changes serialized output consumed outside the package: core/paragraph no longer receives unsupported style.dimensions.maxWidth or matching inline max-width. The effective width remains preserved by the existing generated geometry carrier class and CSS. Supported paragraph styles, author classes, deterministic output, and public APIs are unchanged.

Verification

• composer test: passed
• git diff --check: passed
• GitHub Composer validate and test: passed
• Live SSI fixture-matrix confirmation remains pending safe Lab reconnection.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode orchestrated by Homeboy
• Model: openai/gpt-5.6-sol
• Used for: Traced fixture-derived paragraph save-shape failures, implemented the generic serializer correction, and added end-to-end transformer regressions with Chris.