worker: validate eval-worker streaming resolve (idle-fraction measurement)

[DerDennisOP]

Follow-up to #465, which built streaming Resolve ahead of the measurement the plan gated it on. Measure the cold nixpkgs-scale resolve-phase per-worker idle fraction before/after streaming, confirm it pays off, and tune MAX_RESOLVE_BATCH (or revert to batched) if it does not.

Minor cleanup to fold in while here: worker_pool/resolver.rs resolve_batch does stream.warnings.clone(); use std::mem::take instead (it returns stream afterward).