feat(measurement): add configurable IRTT reflector measurements

Support optional IRTT measurements alongside ICMP with CSV and inline reflector definitions, repeated UCI registration, authentication, and endpoint-specific stream tracking.

Keep the managed driver running during target acknowledgements and stop and join it on event-loop exits. Carry stream identity through baselines, selection, and rate control so endpoints sharing an IP remain distinct.

src/measurement/mod.rs

@@ -1,2 +1,4 @@
 pub(crate) mod icmp;
+#[cfg(feature = "irtt")]
+pub(crate) mod irtt;
 pub(crate) mod model;