Add source fragment conversion contract

[chubes4]

Summary

• Add bfb_convert_fragment() for standalone HTML fragment conversion to serialized block markup.
• Return fragment-scoped diagnostics and provenance for targeted update workflows.
• Expose block-format-bridge/convert-fragment ability and smoke coverage for hero/section fragments plus scoped fallback diagnostics.

Fixes #92.

Testing

• php -l includes/api.php
• php -l includes/abilities.php
• php -l tests/BFBConversionUnitTest.php
• homeboy test --path /Users/chubes/Developer/block-format-bridge@source-fragment-conversion
• homeboy lint --path /Users/chubes/Developer/block-format-bridge@source-fragment-conversion

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: Implemented the source-fragment conversion contract, ability surface, and smoke coverage; Chris remains responsible for review and testing.