Fix Bifrost diagnostics and reference report reproduction

[DavidBakerEffendi]

Bifrost structured scan failures lost their reason and source context in UsageBench reports. Preserve those details without changing failure scoring. Reference-image publication also failed reproduction when the same verified image appeared under an immutable registry reference and a local tag; compare those forms using their equal validated image digests.

Key Changes:

• Retain current and legacy Bifrost scan failure diagnostics.
• Accept differing display references only with equal valid SHA-256 image digests; preserve differing immutable references and digest failures. Add positive and near-miss regressions.

Touch Points:

• src/runners/bifrost.rs
• src/runners/report_compare.rs

Validation: 278 Rust tests and all 59 corpus documents passed before the comparator patch; 12 focused comparator tests and 2 reference-image tests passed after it. Affected-file formatting and diff checks passed. Repository-wide formatting reports pre-existing drift in src/promotion.rs, reproduced on the base.

The reference comparison failure was verified on exact base 3cd9907a4fd6e0c44d1ff6172e13647674e7cf74, main run 37283922007, for both gopls and Bifrost after passing smoke tests.