fix(cli): clamp the benchmark overhead sample at zero

Overhead is wall time minus the sum of the phase deltas. The unattributed
preamble keeps the remainder positive in practice, but summing many small
phase values can round just above the wall measurement, and a negative
overhead in the report would read as an accounting bug.

Author: pablodeymo

bin/ethlambda/src/benchmark/mod.rs

@@ -182,7 +182,11 @@ fn run_synthetic(options: SyntheticOptions) -> eyre::Result<()> {
         );
 
         if measured {
-            let overhead_seconds = wall_seconds - phases.values().sum::<f64>();
+            // Clamped: the unattributed preamble makes the remainder
+            // positive in practice, but summing many small phase values can
+            // round just above the wall measurement, and a negative overhead
+            // in the report would read as an accounting bug.
+            let overhead_seconds = (wall_seconds - phases.values().sum::<f64>()).max(0.0);
             samples.push(Sample {
                 iteration: slot - options.warmup_slots,
                 slot,