Fix layout shell editor geometry

[chubes4]

Summary

• Preserve real Gutenberg layout-shell geometry by marking the single owning InnerBlocks layer with useInnerBlocksProps and flattening only that marker; authored wrapper boxes and nested native block carriers remain intact.
• Project authored position declarations into .editor-styles-wrapper with editor-specific specificity, so Gutenberg core positioning defaults cannot replace authored absolute/relative geometry.
• Preserve frontend save output: the editor-only carrier and marker are not emitted by InnerBlocks.Content.
• Add Chromium anchor-paired coverage for an absolute background/content sibling pair, marker box-neutrality, and native child-carrier preservation.
• Install PHP dependencies in the generic visual-parity CI job, allowing the transformer-backed browser regression to run in CI.

Real Gutenberg Evidence

The #1540 issue workflow exercised the generated Latonia Hope import in a real Gutenberg desktop canvas across Home, About, Gallery, and Resumes. The workflow verified each route after save/reload and performed representative text, media, and link selection/edit/save/reload operations. This change retains the hero absolute-background/content sibling geometry at the matched 1160px content viewport while keeping the frontend output unchanged.

Verification

• composer --working-dir=php-transformer test
• npm test in php-transformer/tools/visual-parity
• git diff --check
• All PHP Transformer and solved-site promotion CI checks passed.

Closes #1540.

Post-merge regression hardening is tracked in #1546 and #1547.

AI Assistance

OpenAI GPT-5.6 Terra through OpenCode reviewed the full trunk diff, implementation topology, test coverage, CI failure, and finalized publication. It made the minimal CI correction required for the transformer-backed browser test, ran verification, and updated this PR. Chris Huber directed the work and remains responsible for the change.