Fix: Restore decimal precision in summary MHz values

[zeroklaw]

Summary

The benchmark summary table and JSON output were truncating MHz values to whole numbers (e.g., 20 instead of 20.596). This was caused by the BenchSummary struct fields being u32 with explicit as u32 casts in get_bench_summary().

Changes

• Changed all BenchSummary MHz fields from u32 → f64 (and Option<u32> → Option<f64>)
• Removed the as u32 truncating casts in both branches of get_bench_summary()

Before

│ Exec Min MHz     │ 18 │
│ Exec Avg MHz     │ 20 │
│ Prove Min MHz    │  1 │

After

│ Exec Min MHz     │ 18.807 │
│ Exec Avg MHz     │ 20.487 │
│ Prove Min MHz    │ 1.874  │

Closes #31