Add raw HTML semantic parity navigation report

[chubes4]

Summary

• Add an HTML semantic parity report for source landmarks and generated block landmarks.
• Report source navigation menus alongside generated core/navigation menus, including item counts, labels, URLs, and core navigation representation.
• Surface semantic parity findings in diagnostics and conversion_report for downstream import diagnostics.
• Add contract coverage for a nonprofit-style seven-item nav pass and a precise missing-core-navigation warning.

Tests

• php tests/contract/run.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implementing the semantic parity report, adding contract tests, and drafting this PR description.