Preserve label/value row geometry

[chubes4]

Summary

• require proven equal-width flex geometry before semantic label/value rows become core/columns
• preserve stacked and content-sized flex rows as native Groups with source-owned CSS
• retain semantic inline markup such as <time>, links, and emphasis through the Group path
• cover the unchanged WP Codebox reproduction and existing faithful Columns behavior

Closes #722.

Verification

• composer test
• 255 parity fixtures
• jq empty php-transformer/tests/fixtures/parity/*.json
• git diff --check

Consumer contract

The WP Codebox #1975 explanatory HTML/CSS fixture was not modified to accommodate this transformer gap. Exact consumer parity remains the downstream acceptance test after this change is released through php-transformer and Static Site Importer.

AI assistance

OpenCode with OpenAI GPT-5.6 Sol implemented and reviewed the geometry classifier, semantic RichText preservation, and regression fixtures under Chris Huber's direction. Chris remains responsible for every line.