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/settings/validation.rs

@@ -3,6 +3,8 @@
 // SPDX-License-Identifier: MPL-2.0
 //
 
+use crate::settings::ReflectorSettings;
+
 use super::{AdvancedSettings, NetworkSettings, ObservabilitySettings, OutputSettings, Settings};
 use std::error::Error;
 use std::fmt::{self, Display, Formatter};
@@ -88,6 +90,7 @@ impl Settings {
         let mut report = ValidationReport::default();
 
         self.network.validate(&mut report);
+        self.reflectors.validate(&mut report);
         self.output.validate(&mut report);
         self.observability.validate(&mut report);
         self.advanced_settings.validate(&mut report);
@@ -192,6 +195,60 @@ impl NetworkSettings {
     }
 }
 
+impl ReflectorSettings {
+    fn validate(&self, report: &mut ValidationReport) {
+        if self.num_reflectors == 0 {
+            report.error("reflectors.num_reflectors", "must be greater than 0");
+        }
+
+        if self.peer_reselection_time == 0 {
+            report.error("reflectors.peer_reselection_time", "must be greater than 0");
+        }
+
+        if self.files.is_empty() && self.entries.is_empty() {
+            report.error(
+                "reflectors",
+                "at least one reflector file or inline entry is required",
+            );
+        }
+
+        for (index, reflector) in self.entries.iter().enumerate() {
+            if reflector.host.trim().is_empty() {
+                report.error(
+                    "reflectors.entries",
+                    format!("entry {index} must have a nonempty host"),
+                );
+            }
+            if reflector.protocols.is_empty() {
+                report.error(
+                    "reflectors.entries",
+                    format!("entry {index} must have at least one measurement protocol"),
+                );
+            }
+        }
+
+        for (index, file) in self.files.iter().enumerate() {
+            if file.trim().is_empty() {
+                report.error(
+                    "reflectors.files",
+                    format!("entry {index} must not be empty"),
+                );
+            }
+        }
+
+        for (index, country) in self.countries.iter().enumerate() {
+            if country.len() != 2 || !country.bytes().all(|c| c.is_ascii_alphabetic()) {
+                report.error(
+                    "reflectors.countries",
+                    format!(
+                        "entry {index} ({country:?}) must be an ISO 3166-1 alpha-2 country code"
+                    ),
+                );
+            }
+        }
+    }
+}
+
 impl OutputSettings {
     fn validate(&self, report: &mut ValidationReport) {
         if self.suppress_statistics {
@@ -298,24 +355,6 @@ impl AdvancedSettings {
             self.tick_interval,
         );
 
-        if self.num_reflectors < 5 {
-            report.error(
-                "advanced_settings.num_reflectors",
-                format!("must be at least 5, got {}", self.num_reflectors,),
-            );
-        }
-
-        if self.peer_reselection_time == 0 {
-            report.error(
-                "advanced_settings.peer_reselection_time",
-                "must be greater than 0",
-            );
-        }
-
-        if self.reflector_list_file.trim().is_empty() {
-            report.error("advanced_settings.reflector_list_file", "must not be empty");
-        }
-
         if self.speed_hist_size == 0 {
             report.error(
                 "advanced_settings.speed_hist_size",