Expose template shell dedupe diagnostics

[chubes4]

Summary

• Add explicit header/footer shell dedupe diagnostic fields for removed block counts, matched template part paths, and preserved local header/footer counts.
• Preserve those fields in compact validation diagnostics.
• Extend smoke coverage to prove global template chrome is removed while local page header/footer content remains.

Tests

• php tests/smoke-template-part-shell-dedupe.php
• php tests/smoke-report-diagnostic-loss-classes.php
• php -l includes/class-static-site-importer-page-materializer.php
• php -l includes/class-static-site-importer-report-diagnostics.php
• php -l tests/smoke-template-part-shell-dedupe.php
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Inspected the dedupe diagnostics gap, implemented report/test hardening, and ran focused verification.