feat(benchmarks): make DaseR prefetch opt-in

[GentleCold]

Summary

• Add --daser-prefetch, defaulting to disabled and enabling two scheduler workers when supplied.
• Preserve the numeric worker override and record effective prefetch settings in manifests and connector config.
• Add focused CLI/manifest/payload coverage and documentation.

FAST'25 evidence

• 200 selected rows, 194 eligible, 6 context-limit skips, 8 in-flight, zero failures in every condition.
• DaseR prefetch off: p50 TTFT 171.56 ms, wall 139.13 s, external-prefix hit rate 31.21%.
• DaseR prefetch on (2 workers): p50 TTFT 208.67 ms (+21.6%), wall 139.91 s, hit rate 31.63% (+0.42 percentage points).
• The predeclared negative-optimization rule is met; default remains off.
• LMCache reference: p50 TTFT 466.20 ms, wall 145.58 s, MP hit rate 30.19%.

Detailed compact artifacts are stored outside the repository under /data/zwt/daser_bench/prefetch_opt_in_fast25_20260905/results.

Tests: 383 passed, 101 deselected; focused 210 passed; ruff passed.