Release main-b12573b

Change units from KHz to MHz (#29)

Closes #28

## Summary

Switch the benchmark output unit from KHz to MHz for all cycle-based
throughput measurements. This aligns with the magnitude typically used
when discussing prover performance.

## Changes

- **`prove_stark`**: Divide by `1_000_000.0` instead of `1000.0` to
produce MHz values. The variable name is updated from `khz` to `mhz`.
- **`BenchResult`**: Rename `exec_khz` → `exec_mhz`, `prove_khz` →
`prove_mhz`. Update `tabled` column headers to `"Exec MHz"` and `"Prove
MHz"`.
- **`BenchSummary`**: Rename all `*_khz` fields to `*_mhz`. Update
column headers accordingly.
- **`prove_snark` comment**: Updated `KHz` → `MHz` in the PostgreSQL
comment.

## Testing

- `cargo build --release -p bento-bench` — passes
- `RISC0_SKIP_BUILD=1 cargo clippy -p bento-bench` — no new warnings
- `RISC0_SKIP_BUILD=1 cargo test -p bento-bench` — 3/3 tests pass