fix(php-transformer): emit a shared navigation entity referenced by token

[chubes4]

Problem

Extracted header chrome still inlined core/navigation-link children. WordPress also ships a default Navigation post with <!-- wp:page-list /-->. Editing the menu in the Site Editor therefore did not change the imported header.

Refs #1945.

Design

After shared-shell extraction, cluster core/navigation blocks that share the same destination list (label + URL, order preserved, including submenus) across template parts and page-owned copies.

Each cluster becomes one plan menus record with inner block_markup, a deterministic token, and reconciliation_identity. Host blocks become self-closing with ref set to {{wordpress-site-plan:navigation:TOKEN}}, keeping overlayMenu, className, style, and layout.

No destination post IDs. SSI persists the entity and substitutes the integer ref.

Fix

• NavigationEntityProjection clusters and rewrites parts/pages, then reanchors runtime entity bindings whose byte offsets moved.
• Bootstrap still emits the current-page navigation-link filter from entity inner markup.

Tests

php tests/contract/shared-navigation-entity.php fails on trunk (Clustered header destinations become one navigation entity) and passes here.

composer test:canonical, composer test:parity (346 fixtures), composer test:packaging, and php tests/contract/production-acceptance-matrix.php passed.

Evidence

Paired SSI dev package (blocks-engine SHA 527302c608eb) imported the Adventuring Ankara capture.

• One wp_navigation post titled Header with Home / Blog / About Me / Contact.
• Default page-list Navigation post remains unused.
• parts/header.html and the homepage navigation are <!-- wp:navigation {"ref":N,"overlayMenu":"mobile",...} /-->.
• 1440 link x: 1122 / 1183 / 1241 / 1329 (unchanged vs baseline).
• 390 hamburger opens the four links.
• Current-page aria-current matches baseline on /, /about-me/, /blog/.
• Editing the entity label updates /, /about-me/, and a post.
• quality_pass with 0 fallbacks.
• Site Editor header canvas: 1 core/navigation, 0 invalid-block warnings.

Merge order

1. This PR (Blocks Engine plan entity + token ref).
2. Automattic/static-site-importer fix/1945-shared-navigation-entity (persist wp_navigation and substitute integer ref).

Do not merge SSI first: older plans have no tokens, and this plan is not yet persisted.

AI assistance

• AI assistance: Yes
• Model: Grok 4.6
• Tool: Claude (Anthropic) via Claude Code, orchestrated by chubes4; follow-up implementation via OpenCode (Grok 4.6)
• Used for: Reading Shared chrome stays inline in post_content when a capture has responsive document variants #1945 and related PRs, emitting the shared navigation entity, adding the trunk-failing contract test, running composer suites, pairing an SSI import, measuring geometry, and drafting this PR. A human remains responsible for review and merge.

Refs #1945