M4.1d — Observability: per-session cost and failure report

[Ferrari-72]

Parent

Spawned from #8 (M4.1 tracking issue).

Objective

Surface per-session cost, failure reasons, and AI error rates so benchmark health can be monitored.

Definition of done

• results/*.json per-result rows include an optional error field when result starts with error:
• build_index.py aggregates error counts into the index entry (error_count, skip_count)
• make test-unit still passes

Dependencies

Depends on #3, #4.