Measure throughput with subsecond precision

[Toti330]

Summary

Replaces whole-second throughput calculation with a precise floating-point elapsed duration.

Why this is useful

A 10,000-seed run that takes anywhere from 60.000 to 60.999 seconds previously displayed the same 166.66667 seeds/sec. This removes that quantization, so throughput comparisons can distinguish real changes from timing truncation.

Implementation

• Uses Duration::as_secs_f64() for the final elapsed duration.
• Prints elapsed seconds and seeds/sec with 6 decimal places.
• Does not change worker, writer, channel, database, checkpoint, or scheduling behavior.

Independence

This PR is based directly on main and is intentionally standalone. It does not require any other performance or diagnostics PR.

Verification

• cargo test --all-targets passed in the Rust Docker toolchain.
• Benchmark smoke run printed 0.208606 elapsed seconds and 38.349896 seeds/sec, rather than an integer-second-derived value.

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Generated by GPT-5.6 Terra & Sol. Ideas from the author; implementation by AI.

[SuperB3333]

Implemented in 83586fe