chi routes only HEAD to the handler, but assert explicitly so a
future router config change cannot silently accept GET/POST and leak
latency bytes or side-effects.
Updates #3157
Unit tests for Change (IsEmpty, Merge, Type, PingNode constructor),
ping tracker (register/complete/cancel lifecycle, concurrency, latency),
and end-to-end servertests exercising the full round-trip with real
controlclient.Direct instances.
Updates #2902
Updates #2129