Prove editor position projection against Gutenberg core

[chubes4]

Summary

• Render the layout-shell browser fixture against Gutenberg's native child-carrier position: relative rule.
• Prove generated editor-scoped authored positioning preserves frontend-equivalent geometry.
• Prove the same render diverges when the production editor position projection is removed.
• Bound the counterfactual transformation to single CSS rules, so it cannot cross braces or remove authored geometry declarations unrelated to position.
• Retain computed-style assertions for authored background inset, content padding, shell geometry, marker neutrality, and saved frontend markup.

Closes #1546.

Verification

• node tests/layout-shell-editor-geometry.mjs in php-transformer/tools/visual-parity
• npm test in php-transformer/tools/visual-parity
• git diff --check

AI Assistance

OpenAI GPT-5.6 Terra through OpenCode reviewed the #1543 regression, implemented the cascade counterfactual, then tightened the follow-up transformation after independent review. It ran focused and visual-parity verification and prepared this follow-up. Chris Huber directed the work and remains responsible for the change.