Preserve semantic template part boundaries

[chubes4]

Summary

• retain separate shell representations for page identity/pruning, template-part content, and compatibility projections
• let wp:template-part own the outer header/footer landmark via semantic tagName
• prevent nested landmark groups inside template-part payloads
• preserve source class, anchor, inline style, and content hooks on a non-landmark wrapper across canonical v2, compiled-site v1, and materialization-plan v1
• keep page pruning based on bare inner markup so extracted chrome is not duplicated in post content

Advances #632.
Cross-references #688 for runtime-bound shell retention safety.

Compatibility

Existing page shell identity and runtime-binding checks continue to use the full landmark representation. Compiled-site and materialization v1 retain their schemas while carrying source presentation on a non-landmark group. Canonical template references add tagName, matching WordPress core template-part rendering semantics.

Verification

• composer --working-dir=php-transformer test
• canonical contracts and all unit suites
• 248 parity fixtures
• package-install proof
• php php-transformer/tests/contract/shared-shell-plan.php
• php php-transformer/tests/contract/wordpress-site-plan.php
• php php-transformer/tests/contract/run.php
• PHP lint and git diff --check

AI assistance

• Model: OpenAI GPT-5.6 Sol
• Tool: OpenCode
• Used for: integration-history isolation, conflict resolution against current trunk, compatibility review, contract strengthening, and test execution with Chris Huber.