concurrency: add round-trip latency percentile test for USPSCQueue

[pjsutar]

Motivation: the existing bench test measures throughput (ops/sec) but omits tail latency. In real-time pipelines such as sensor DAQ systems and market data handlers, p99/p999 latency is the binding constraint.

Changes:

• Add UnboundedQueue.RoundTripLatencyPercentiles test
• Producer enqueues send timestamp as payload; consumer echoes it back
• Measures p50/p95/p99/p999 round-trip latency in nanoseconds
• Uses steady_clock (monotonic) to avoid wall-clock skew
• Includes 1000-iteration warmup before recording measurements
• Sanity assertions verify positive and monotonically ordered percentiles

Sample output (WSL2, Intel Core i7, gcc 13):
p50=309ns p95=413ns p99=516ns p999=1134ns

Tested: gcc 13, Ubuntu 22.04 LTS (WSL2), x86_64

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