Verify parser-backed navigation snapshots

[jt55401]

Summary

• parse signed sections with the browser HTML parser when available
• balance nested sections in the strict Node fallback
• compare frozen source nodes with rendered nodes using separate source and rendered base URLs
• include profile and signature scope in snapshot staleness checks

Verification

• npm run typecheck
• npm test (39 tests)
• npm run build

This prepares the client lifecycle for the v1 signing-payload migration.