feat(measurement): integrate IRTT and configurable reflector selection

Add protocol-aware reflector definitions, TOML/UCI lists, validation and environment overrides. Resolve and select endpoint-aware measurement streams, honor configured ICMP timing and socket routing, and retain partial-set reselection behavior from the standalone fixes.

Drive the managed IRTT task throughout its lifecycle, retry terminal targets without restarting healthy sessions, normalize blank HMAC keys and use effective RTT for latency fallback. Fold the reviewed configuration, resolution and measurement corrections into their implementation.

src/measurement/icmp.rs

@@ -326,6 +326,7 @@ impl IcmpEngine {
                     stream: probe.stream,
                     started_at: probe.sent_at,
                     observed_at,
+                    round_trip: None,
                     one_way: None,
                 })
             }
@@ -385,6 +386,7 @@ impl IcmpEngine {
                     stream: probe.stream,
                     started_at: probe.sent_at,
                     observed_at,
+                    round_trip: None,
                     one_way: Some(OneWayLatency {
                         uplink: SignedDuration::from_millis(uplink_ms),
                         downlink: SignedDuration::from_millis(downlink_ms),