Improve generic navigation pattern recognition

[chubes4]

Summary

• Recognize labeled footer/mobile link clusters as navigation sections when the wrapper itself has no nav class.
• Strip decorative SVGs and empty icon spans from generated navigation labels.
• Add parity fixtures based on SSI matrix header/footer navigation shapes.

Tests

• composer test:canonical
• composer test:parity
• composer test

Evidence

• Matrix run context: d8f80380-5220-4d18-8ccc-0ee2cacd1680 reported 71/71 failed with runtime_target_gap/static_site_import_quality findings.
• Before targeted snippet at d04a0c2: decorative labels retained nav-chevron/dropdown__dot markup; footer labeled link clusters produced 0 core/navigation blocks and kept footer links as paragraph content.
• After this branch: decorative labels become clean text labels and remove nav-chevron/dropdown__dot from serialized navigation; footer/mobile labeled link clusters produce 2 core/navigation blocks and no footer-link paragraph blob.

AI assistance

• AI assistance: Yes
• Tool(s): gpt-5.5 via OpenCode
• Used for: Inspecting transformer patterns, implementing generic navigation/header/footer recognition improvements, adding parity fixtures, and running local verification.