feat(mesh): show federated Archipelago nodes on the Mesh Map

Peers that opt in via a new "Share Location" toggle in Settings
(server.set-location RPC) get plotted on other trusted peers' Mesh Map
with a distinct Archy-logo marker, separate from raw LoRa radio peers.
Location is persisted locally, carried in NodeStateSnapshot, and
propagated through federation sync/delta like other node state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

core/archipelago/src/transport/delta.rs

@@ -48,6 +48,12 @@ pub struct StateDelta {
     /// Tor active flag (only if changed).
     #[serde(skip_serializing_if = "Option::is_none")]
     pub tor: Option<bool>,
+    /// Shared location (only if changed) — same "None means unchanged"
+    /// convention as the other scalar fields here.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub lat: Option<f64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub lon: Option<f64>,
 }
 
 /// Compute the delta between two state snapshots.
@@ -114,6 +120,12 @@ pub fn compute_delta(prev: &NodeStateSnapshot, curr: &NodeStateSnapshot) -> Stat
     if curr.tor_active != prev.tor_active {
         delta.tor = curr.tor_active;
     }
+    if curr.lat != prev.lat {
+        delta.lat = curr.lat;
+    }
+    if curr.lon != prev.lon {
+        delta.lon = curr.lon;
+    }
 
     delta
 }
@@ -162,6 +174,12 @@ pub fn apply_delta(base: &NodeStateSnapshot, delta: &StateDelta) -> NodeStateSna
     if let Some(tor) = delta.tor {
         result.tor_active = Some(tor);
     }
+    if let Some(lat) = delta.lat {
+        result.lat = Some(lat);
+    }
+    if let Some(lon) = delta.lon {
+        result.lon = Some(lon);
+    }
 
     result
 }
@@ -225,6 +243,8 @@ mod tests {
             nostr_npub: None,
             own_fips_npub: None,
             federated_peers: Vec::new(),
+            lat: None,
+            lon: None,
         }
     }
 
@@ -259,6 +279,8 @@ mod tests {
             nostr_npub: None,
             own_fips_npub: None,
             federated_peers: Vec::new(),
+            lat: None,
+            lon: None,
         }
     }