model-bench: warn when arm and judge share a model family #391

model-bench never warns when an arm's model family equals the judge's (model-bench-diff.py:16,107). Add the warning plus a test.

From the 2026-10-03 pstack-claude adoption audit (docs/research/pstack-claude-audit-2026-10-03.md, PR #385). Constraints: bash 3.2 and Python 3.9, failing test first (Rule 4), no hardcoded home paths, stage by name.