Extract fallback finding normalization

[chubes4]

Summary

• Extract fallback generic metadata enrichment from FallbackDiagnostic into FallbackFindingNormalizer.
• Keep FallbackDiagnostic focused on diagnostic-code default payloads while preserving the existing public build/withGenericFindingMetadata API.
• Add focused unit coverage for unsupported-element family normalization, selector specificity projection, ancestry reasons, and inline semantic runtime-island classification.

Testing

• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Inspected the PHP transformer diagnostics flow, implemented the behavior-preserving extraction, added focused unit coverage, ran tests, and prepared this PR.