Self-checking function + performance harness with tail-latency percentiles

The psytest suite was rebuilt into a context-chained set of stages exercising context switching, time sync, guaranteed & best-effort messaging, signals, and catalog retrieval — each stage emitting a PASS/FAIL + perf line with a final aggregated verdict. It now reports full p50/p90/p95/p99/p99.9 tail-latency percentiles, not just averages.