Retain entry shell when shared-part extraction is ambiguous

[chubes4]

Problem

Artifact compilation removes the entry page header/footer before WordPressSitePlan decides whether those shell candidates are canonical shared template parts. When shared-shell extraction is rejected as ambiguous, no template part is emitted and the entry page has already lost its shell. Fixture 37-art-gallery-exhibition reproduces this with wordpress_site_plan_shell_retained_ambiguous: compiled index.html has an empty .site-header and no source footer.

The relevant ordering is in php-transformer/src/ArtifactCompiler/ArtifactCompiler.php: entry conversion enables shell extraction, then compiledSiteReport() removes extracted entry shell markup before the site-plan result is known. Existing shared-shell tests cover ambiguous retention in page content but not this entry-removal sequence.

Work

• Decide canonical shared parts before removing entry shell markup, or restore original entry shell whenever extraction is declined.
• Preserve exactly one header/footer per route whether the shell becomes template parts or remains page content.
• Keep runtime-bound or otherwise non-extractable shell content intact.
• Emit explicit provenance for accepted versus rejected shell extraction.

Acceptance

• A multipage ambiguous-shell fixture retains exactly one header/footer on every route and emits no template part.
• A canonical shared-shell fixture still emits template parts without duplicate page shells.
• Fixture 37 no longer loses the entry shell for this reason.
• PHP transformer contract and parity suites pass.

Discovered through parallel GPT-5.6 Sol analysis in OpenCode; Chris owns the issue and acceptance criteria.