refactor: replace legacy Config with Settings

Remove legacy src/config.rs and migrate all application consumers to
the nested Settings model. Baseliner, metrics, pingers, ratecontroller,
reflector selector, and main now use Settings. Derived min rates become
methods on NetworkSettings rather than stored legacy fields.
MeasurementType and ObservabilityProtocol move into the settings module.

src/metrics.rs

@@ -2,8 +2,8 @@
 //
 // SPDX-License-Identifier: MPL-2.0
 
-use crate::Config;
-use crate::config::{MeasurementType, ObservabilityProtocol};
+use crate::settings::Settings;
+use crate::settings::{MeasurementType, ObservabilityProtocol};
 use crate::time::Time;
 use flume::{Receiver, RecvTimeoutError, Sender};
 use log::{error, info, warn};
@@ -157,14 +157,14 @@ impl MetricsSender {
 }
 
 pub struct Metrics {
-    pub config: Config,
+    pub settings: Settings,
     pub metrics_dropped: Arc<AtomicU32>,
     pub metrics_rx: Receiver<(Metric, u64)>,
 }
 
 impl Metrics {
     pub fn run(self) -> anyhow::Result<()> {
-        let host = match &self.config.observability_host {
+        let host = match &self.settings.observability.host {
             Some(h) => h.clone(),
             None => {
                 error!("Observability host not configured - metrics exporter disabled");
@@ -172,12 +172,12 @@ impl Metrics {
             }
         };
 
-        let port = self.config.observability_port;
-        let batch_size = self.config.observability_batch_size;
-        let timeout = Duration::from_millis(self.config.observability_batch_timeout_ms);
-        let host_tag = &self.config.observability_host_tag.clone();
+        let port = self.settings.observability.port;
+        let batch_size = self.settings.observability.batch_size;
+        let timeout = Duration::from_millis(self.settings.observability.batch_timeout_ms);
+        let host_tag = &self.settings.observability.host_tag.clone();
 
-        let mut transport = match self.config.observability_protocol {
+        let mut transport = match self.settings.observability.protocol {
             ObservabilityProtocol::Udp => {
                 info!("Metrics exporter configured for UDP to {}:{}", host, port);
                 Transport::new_udp(&host, port)?