Add --benchmark_cooldown flag to mitigate thermal throttling

Add --benchmark_cooldown flag to mitigate thermal throttling

Adds a --benchmark_cooldown=<seconds> flag that inserts a sleep between
benchmark families. This helps produce consistent results on systems
where sustained workloads cause thermal throttling and CPU frequency
scaling.

Uses a wrapping BenchmarkReporter that sleeps before forwarding results
to the default display reporter.