measurement: adapt the echo probe rate to loss and RTT, and weight samples by the time they stand for

udp_echo and tcp_echo run at the lowest rate that still proves the loss bound and keeps the RTT p99 confidence interval within tolerance, go to the maximum when the line loses packets or its RTT rises, and back off exponentially when it is unreachable. Every echo row carries a weight (the time it stands for, in grid intervals) and the analysis ops accumulate loss and RTT by it, so p50/p95/p99 and loss stay time-averaged when the rate changes. State changes are logged for audit.

backend/agent/src/MoShiro.Agent/DataPlane/Registries.cs

@@ -217,7 +217,7 @@ public sealed class StreamInfo
         KindSpec spec = Kinds.Find(kind)!;
         DeclaredPps = kind switch
         {
-            "udp_echo" or "tcp_echo" => parameters.GetInt("rate_hz"),
+            "udp_echo" or "tcp_echo" => Kinds.PeakRateHz(kind, parameters),
             "udp_train" => parameters.GetNumber("mbps") * 1_000_000 / 8 / Math.Max(1, parameters.GetInt("size")),
             _ => 20,
         };