Preserve editable projected layout shells

[chubes4]

Summary

• compress exact projected wrapper branches deterministically, independent of unrelated document depth
• preserve serializable authored styles and semantic template-part extraction through layout shells
• add concise read-only List View and breadcrumb labels
• isolate Gutenberg block props on a box-neutral editor carrier so editor CSS cannot override authored positioning

Progresses #868.

Validation

• cd php-transformer && composer test
• canonical and unit contracts passed
• 289 PHP transformer parity fixtures passed
• package installation proof passed
• representative artifact: 562 to 504 blocks, 262 to 146 Groups, and depth 18 to 14
• Gutenberg runtime: 164 box-neutral layout-shell carriers, zero authored background wrappers carrying Gutenberg layout classes, three large media layers with nonzero geometry, stable layout-shell selection, and no console errors
• frontend runtime geometry remained unchanged

Follow-up

Two duplicated responsive copies of one source form remain provider-materializable fallbacks, matching the existing baseline gap. Wrapper projection absorbs the original form Group after its runtime binding is created; final binding resolution cannot find that pre-projection block identity, so the form declarations are omitted and provider materialization does not run. A focused follow-up will rebase runtime bindings onto final projected block identities after structural rewrites.

AI Assistance

OpenAI GPT-5.6 Sol via OpenCode was used to inspect the transformer and Gutenberg runtime, implement and test the layout-shell changes, run the representative import and browser verification, and draft this pull request. Chris Huber directed the architecture and acceptance criteria and reviewed the resulting site behavior.