Fix lost Bifrost incomplete scan diagnostics

[DavidBakerEffendi]

Bifrost reports detailed enumeration evidence in incomplete scan notes, but UsageBench discarded it and retained only semantic_analysis. Preserve those notes in report diagnostics so failures retain their underlying evidence.

Key Changes:

• Include incomplete scan notes without changing locations, partial status, or scoring.
• Add a regression covering a valid location returned under incomplete enumeration.

Touch Points:

• src/runners/bifrost.rs

Validation: 284 Rust tests pass; all 59 benchmark documents validate; touched-file formatting and diff checks pass.