Test resolved grid layout semantics

[chubes4]

Summary

• lock inline CSS grid containers to core/group rather than core/columns
• cover the same precedence when grid declarations come from resolved stylesheet CSS
• protect the generic grid guard landed in Preserve resolved grid layout semantics #621 against regression

Closes #620.

How to test

1. Check out this branch on a fresh clone.
2. Run composer --working-dir=php-transformer test:parity.
3. Confirm all 243 parity fixtures pass.
4. Run composer --working-dir=php-transformer test.
5. Confirm the PHP transformer contracts and package-install proof pass.

Backwards compatibility

This is regression coverage only and does not change runtime behavior or extension paths. It codifies the behavior already shipped by #621: source CSS grid geometry takes precedence over the heuristic Columns conversion.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-sol via OpenCode
• Used for: Investigated representative SSI failures, drafted parity fixtures, and verified the transformer suites. Chris remains responsible for every line.