Preserve runtime entity binding anchors through shared-shell extraction

[chubes4]

Problem

The canonical WordPress site-plan compiler can generate runtime entity bindings for a commerce/forms artifact, then remove the binding’s declared page anchor during shared-shell extraction. WordPressSitePlan::assertEntityBindingsRemainPageOwned() correctly rejects the resulting detached binding, so no canonical site plan is produced.

This blocks the WooCommerce interoperability fixture used by Automattic/wp-codebox#1974. WooCommerce and Jetpack provision successfully before compilation; this is not a plugin dependency failure.

Reproduction

Fixture: fixtures/websites/20-switchback-woocommerce-extra-hard

Revisions:

• Blocks Engine: 2adb310a6094949d206eaaaeebe43bb08d42c559
• Static Site Importer: 9414dc69ec8e44cf42e34a37c4db59c5b14a5331
• Homeboy run: ae30feeb-062e-4f3f-aa21-8bef32d08f2b

node tools/run-fixture-matrix.mjs \
  --runner homeboy-lab \
  --static-site-importer /path/to/static-site-importer \
  --blocks-engine /path/to/blocks-engine \
  --target-fixture 20-switchback-woocommerce-extra-hard \
  --batch-size 1 \
  --concurrency 1 \
  --run-id wp-codebox-1974-woocommerce-9414dc69-2adb310a

Observed error:

wordpress_site_plan_not_self_contained
A runtime entity binding no longer has its declared source-page block anchor after shell extraction.

The failure originates at php-transformer/src/WordPressSitePlan/WordPressSitePlan.php after sharedShells() rewrites pages and before plan construction.

Expected

Shared-shell extraction must preserve entity ownership deterministically. A binding anchored in markup moved to a shared template part should either:

• remain page-owned through a retained page anchor, or
• be canonically rebound to the extracted owning surface with corresponding schema/validation semantics.

Acceptance

• the fixture compiles a valid blocks-engine/wordpress-site-plan/v2
• every runtime entity binding remains attached to canonical materialized markup
• product/form provider bindings still seed and replace the intended blocks
• WordPressSitePlan::assertValid() remains fail-closed for truly detached bindings
• add deterministic contract coverage for bindings whose candidate markup participates in shared-shell extraction