Expose structured fallback diagnostics

[chubes4]

Summary

• Adds generic fallback_diagnostics entries to HTML conversion reports so callers can inspect unsupported fragments without parsing serialized block comments.
• Enriches fallback events with reason code, source tag, useful attributes/classes, byte preview, and generated block type.
• Covers iframe, form, and custom-element fallback diagnostics in conversion tests.

Fixes #113

Tests

• homeboy test --path /Users/chubes/Developer/block-format-bridge@fix-issue-113-fallback-diagnostics --extension wordpress --skip-lint -- tests/BFBConversionUnitTest.php --filter '/fallback|diagnostic/i'
• homeboy lint --path /Users/chubes/Developer/block-format-bridge@fix-issue-113-fallback-diagnostics --extension wordpress

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the diagnostics implementation and targeted tests; Chris remains responsible for review and merge.