XY-953 Add proactive brief benchmark scoring

[yvette-carlisle]

Summary

• Add proactive brief benchmark fixtures and scoring for daily project brief, resume-work brief, stale decision audit, stale plan/preference warning, and private-corpus refresh blocker.
• Extend the real-world job benchmark with evidence refs, freshness/currentness markers, action rationale, reject/defer reasons, and unsupported/tombstone violation guards.
• Update the XY-951 stage ledger and benchmarking docs with the proactive brief stage result: improved, 4 pass / 1 blocked, 0 wrong-result/unsupported/tombstone violations.

Validation

• cargo make real-world-memory-proactive-brief
• cargo test -p elf-eval --test real_world_job_benchmark proactive_brief -- --test-threads=1
• cargo test -p elf-eval --test real_world_job_benchmark -- --test-threads=1
• cargo make real-world-memory
• cargo make fmt
• cargo make lint-fix
• cargo make checks

Notes

• The private-corpus refresh case remains a typed blocker pending live private-corpus inputs (tracked separately by XY-930).