bench: add MTP options to ds4-bench

[tnhnyc]

Summary

• add --mtp, --mtp-draft, and --mtp-margin to ds4-bench
• route the benchmark generation loop through the existing greedy MTP path when draft depth is greater than one
• document a minimal normal-vs-MTP benchmark invocation and add an opt-in DS4_MTP_VERIFY_TIMING breakdown for verifier profiling

Overlap check

I checked the current issue/PR list before opening this. Related discussions exist around speculative decoding (#80, #63) and the large CUDA/backend PR #187 also mentions speculative-decoding plumbing, but I did not find a focused PR that makes ds4-bench directly benchmark the existing MTP path on main. This PR is intentionally small and measurement-oriented; it does not change default runtime behavior.

Testing

• make
• git diff --check origin/main..HEAD
• ./ds4-bench --help shows the new MTP options