Separate product-neutral responsive layout companions

[chubes4]

Summary

• emit a dedicated responsive-layout companion block instead of overloading responsive-media with kind: layout
• move the WordPress companion payload and renderer capabilities into the producer-owned blocks-engine/* namespace
• keep responsive-media rendering narrow while bounding and sanitizing static layout capture
• remove executable contract knowledge of Static Site Importer from Blocks Engine

Closes #1143

Consumer: Automattic/static-site-importer#1301

Verification

• composer test in php-transformer
• 287 parity fixtures
• package-install proof
• composer validate --strict
• git diff --check

AI assistance

OpenCode with OpenAI gpt-5.6-sol traced the producer/consumer ownership leak, separated responsive layout from responsive media, migrated contracts to producer-owned identifiers, and ran the full PHP Transformer suite. Chris Huber directed the architecture and remains responsible for the change.

[aagam-shah]

Ran this branch (ae9716ef) with Automattic/static-site-importer#1301 on a real 28-page Wix store artifact. The captured content is no longer swallowed, but the plan is stopped by two policy gates before any page renders:

• editability policy (enforcement: required): max_nesting_depth 26 vs maximum 20 on 10 pages (36 failures) — Wix wraps <main> in 21+ divs;
• SSI quality gate: 2 interactive_form fallbacks for the contact form (provider_materializable_form).

The composed plan holds 2 custom/responsive-layout, 54 custom/responsive-media, 1,614 custom/layout-shell, and 58 core/html blocks. Shared plan grew from 39 MB on trunk to 231 MB here; compile_pages went from 427 s to 728 s (overlapping runs, so treat timings as rough).

Full numbers and the trunk comparison: Automattic/static-site-importer#1301 (comment)

AI assistance: Claude Code (Claude Fable 5) ran the comparison and drafted this comment. Aagam Shah directed the work and is responsible for it.