feat(bench): LMEVAL_VARIANT env override for experiment isolation in Modal harness #1128

What

Add an optional LMEVAL_VARIANT env var (or --variant flag) to modal_fast50.py that appends a custom suffix to the volume variant key.

Why

The volume resume feature keys results by (strategy, temporal, mmr_lambda, question_id). This is sufficient for flag-driven experiments, but not for code-driven experiments: tweaking retrieval code (e.g. temporal window logic in temporal.py) without adding a new flag produces the same variant key, so a resumed run silently returns stale results computed by the old code — the exact failure mode the variant-keyed paths were introduced to prevent (see review finding on #1127).

Today the only workaround is manually clearing the Modal volume, which also destroys valid resume state for other variants.

Testing

• LMEVAL_VARIANT= unset → behaviour identical to today (no suffix)
• LMEVAL_VARIANT=exp1 → volume path includes suffix; resume scoped to it; baseline variants untouched
• compare_fast50.py baseline exp1 still diffs correctly (outdir already namespaced)