Prioritize stored agreed range over default range in reserve_range

When an agent restarted with multiple networks (e.g. hexor and ultradesu),
Agent::reserve_range() previously attempted to reserve the default configured
range (10.13.37.0/24) for whichever network happened to be listed first in
storage, even if that network had already agreed upon a different range
(such as 10.79.175.0/24). As a result, the first network seized 10.13.37.0/24
in the route table, causing the second network (which genuinely used 10.13.37.0/24)
to trigger a spurious RouteError::Overlap warning at startup until the first
network asynchronously loaded its signed records and released the reservation.

Key changes:
- In crates/tsunagi/src/agent/mod.rs, make reserve_range asynchronous and
  inspect stored signed records first using StateSet::agreed_range().
- If an agreed range already exists in storage, reserve that specific range
  in routes immediately instead of preemptively claiming the default
  overlay_ipv4_range.

Validation on Linux:
- cargo fmt --all -- --check passed.
- cargo clippy --locked --workspace --all-targets -- -D warnings passed.
- cargo test --locked --workspace --all-targets --jobs 2 -- --test-threads=2 passed (349 tests passed).

crates/tsunagi/src/agent/mod.rs

@@ -385,10 +385,7 @@ impl Agent {
     /// waits to adopt whatever it settles on. One agent has one interface, so
     /// proposing a range it could not route would be worse than having none:
     /// the lowest author's range wins, and the collision would spread.
-    fn reserve_range(&self, network: NetworkId) -> RangePlan {
-        let Some(configured) = self.inner.config.overlay_ipv4_range else {
-            return RangePlan::default();
-        };
+    async fn reserve_range(&self, network: NetworkId) -> RangePlan {
         let reserve = |wanted: Ipv4Range| {
             let reservation = crate::overlay::NetworkRoutes {
                 broadcast: Default::default(),
@@ -399,6 +396,44 @@ impl Agent {
             self.inner.routes.set_network(network, reservation).is_ok()
         };
 
+        // If this network already has signed records in storage, check if its
+        // members have already agreed on a range. If so, preserve and reserve
+        // THAT range rather than preemptively seizing the default configured range.
+        let stored_records = self
+            .inner
+            .storage
+            .signed_records(network)
+            .await
+            .unwrap_or_default();
+        let mut state_set = crate::state::StateSet::new();
+        state_set.merge_all(network, stored_records);
+        if let Some(agreed) = state_set.agreed_range() {
+            if reserve(agreed) {
+                return RangePlan {
+                    propose: if Some(agreed) == self.inner.config.overlay_ipv4_range {
+                        Some(agreed)
+                    } else {
+                        None
+                    },
+                    ..RangePlan::default()
+                };
+            } else {
+                tracing::warn!(
+                    %network,
+                    range = %agreed,
+                    "agreed range for this network overlaps another network already active on this interface"
+                );
+                return RangePlan {
+                    conflict: Some(agreed),
+                    ..RangePlan::default()
+                };
+            }
+        }
+
+        let Some(configured) = self.inner.config.overlay_ipv4_range else {
+            return RangePlan::default();
+        };
+
         // The configured range first, so a network a device has always had
         // keeps the addresses it has always had.
         if reserve(configured) {
@@ -610,7 +645,7 @@ impl Agent {
             return Err(Error::NetworkAlreadyActive(network_id));
         }
 
-        let range = self.reserve_range(network_id);
+        let range = self.reserve_range(network_id).await;
         let mut backends = Vec::new();
         if let Some(discovery) = &self.inner.config.discovery {
             backends.push(discovery.clone());