COORDS_PRESENT warmup-then-reactive for DataPackets

Include source and destination coordinates on the first N DataPackets
of each session (default 5, configurable via
node.session.coords_warmup_packets). This warms transit node
coord_caches so multi-hop forwarding and error signal routing work
after SessionSetup cache entries expire.

On CoordsRequired receipt, reset the counter to re-enable coordinate
inclusion for the next N packets, handling mid-session cache expiry
and path changes.

Changes:
- SessionConfig: add coords_warmup_packets field (default 5)
- SessionEntry: add coords_warmup_remaining counter, initialized on
  Established transition (both initiator and responder paths)
- send_session_data(): attach coords via DataPacket::with_coords()
  while counter > 0, decrement per send
- handle_coords_required(): reset counter for affected session
- 4 new unit tests for counter lifecycle and config default

src/node/handlers/session.rs

@@ -211,6 +211,7 @@ impl Node {
         };
 
         entry.set_state(EndToEndState::Established(session));
+        entry.set_coords_warmup_remaining(self.config.node.session.coords_warmup_packets);
         entry.touch(Self::now_ms());
         self.sessions.insert(*src_addr, entry);
 
@@ -278,6 +279,7 @@ impl Node {
                 }
             };
             entry.set_state(EndToEndState::Established(noise_session));
+            entry.set_coords_warmup_remaining(self.config.node.session.coords_warmup_packets);
             debug!(src = %src_addr, "Session established (responder, on first data)");
         }
 
@@ -341,6 +343,18 @@ impl Node {
         );
 
         self.maybe_initiate_lookup(&msg.dest_addr).await;
+
+        // Reset coords warmup counter so the next N packets include
+        // COORDS_PRESENT, re-warming transit caches along the path.
+        if let Some(entry) = self.sessions.get_mut(&msg.dest_addr) {
+            let n = self.config.node.session.coords_warmup_packets;
+            entry.set_coords_warmup_remaining(n);
+            debug!(
+                dest = %msg.dest_addr,
+                warmup_packets = n,
+                "Reset coords warmup counter after CoordsRequired"
+            );
+        }
     }
 
     /// Handle a PathBroken error signal from a transit router.
@@ -449,8 +463,20 @@ impl Node {
             reason: format!("session encrypt failed: {}", e),
         })?;
 
-        // Build DataPacket and wrap in SessionDatagram
-        let data_packet = DataPacket::new(ciphertext);
+        // Check warmup counter and decrement (while entry is still borrowed)
+        let include_coords = entry.coords_warmup_remaining() > 0;
+        if include_coords {
+            entry.set_coords_warmup_remaining(entry.coords_warmup_remaining() - 1);
+        }
+
+        // Build DataPacket, conditionally with coordinates
+        let mut data_packet = DataPacket::new(ciphertext);
+        if include_coords {
+            let my_coords = self.tree_state.my_coords().clone();
+            let dest_coords = self.get_dest_coords(dest_addr);
+            data_packet = data_packet.with_coords(my_coords, dest_coords);
+        }
+
         let my_addr = *self.node_addr();
         let datagram = SessionDatagram::new(my_addr, *dest_addr, data_packet.encode())
             .with_hop_limit(self.config.node.session.default_hop_limit);

src/node/session.rs

@@ -54,6 +54,10 @@ pub(crate) struct SessionEntry {
     created_at: u64,
     /// Last activity timestamp (Unix milliseconds).
     last_activity: u64,
+    /// Remaining DataPackets that should include COORDS_PRESENT.
+    /// Initialized from config when session becomes Established;
+    /// reset on CoordsRequired receipt.
+    coords_warmup_remaining: u8,
 }
 
 impl SessionEntry {
@@ -70,6 +74,7 @@ impl SessionEntry {
             state: Some(state),
             created_at: now_ms,
             last_activity: now_ms,
+            coords_warmup_remaining: 0,
         }
     }
 
@@ -116,4 +121,15 @@ impl SessionEntry {
     pub(crate) fn last_activity(&self) -> u64 {
         self.last_activity
     }
+
+    /// Remaining DataPackets that should include COORDS_PRESENT.
+    pub(crate) fn coords_warmup_remaining(&self) -> u8 {
+        self.coords_warmup_remaining
+    }
+
+    /// Set the coords warmup counter (used on Established transition
+    /// and CoordsRequired reset).
+    pub(crate) fn set_coords_warmup_remaining(&mut self, value: u8) {
+        self.coords_warmup_remaining = value;
+    }
 }

src/node/tests/session.rs

@@ -1299,6 +1299,90 @@ fn test_purge_idle_sessions_disabled_when_zero() {
     assert_eq!(node.session_count(), 1, "Sessions should not be purged when idle timeout is disabled");
 }
 
+// ============================================================================
+// Unit tests: COORDS_PRESENT warmup counter
+// ============================================================================
+
+#[test]
+fn test_coords_warmup_counter_default_zero_on_new() {
+    use crate::noise::HandshakeState;
+
+    let identity_a = Identity::generate();
+    let identity_b = Identity::generate();
+
+    let handshake = HandshakeState::new_initiator(
+        identity_a.keypair(),
+        identity_b.pubkey_full(),
+    );
+
+    let entry = crate::node::session::SessionEntry::new(
+        *identity_b.node_addr(),
+        identity_b.pubkey_full(),
+        EndToEndState::Initiating(handshake),
+        1000,
+    );
+
+    assert_eq!(entry.coords_warmup_remaining(), 0,
+        "Counter should be 0 for non-Established sessions");
+}
+
+#[test]
+fn test_coords_warmup_counter_set_and_get() {
+    let node = make_node();
+    let remote = Identity::generate();
+    let remote_addr = *remote.node_addr();
+
+    let session = make_noise_session(node.identity(), &remote);
+    let mut entry = crate::node::session::SessionEntry::new(
+        remote_addr,
+        remote.pubkey_full(),
+        EndToEndState::Established(session),
+        1000,
+    );
+
+    assert_eq!(entry.coords_warmup_remaining(), 0);
+
+    entry.set_coords_warmup_remaining(5);
+    assert_eq!(entry.coords_warmup_remaining(), 5);
+
+    entry.set_coords_warmup_remaining(0);
+    assert_eq!(entry.coords_warmup_remaining(), 0);
+}
+
+#[test]
+fn test_coords_warmup_counter_decrement() {
+    let node = make_node();
+    let remote = Identity::generate();
+    let remote_addr = *remote.node_addr();
+
+    let session = make_noise_session(node.identity(), &remote);
+    let mut entry = crate::node::session::SessionEntry::new(
+        remote_addr,
+        remote.pubkey_full(),
+        EndToEndState::Established(session),
+        1000,
+    );
+
+    entry.set_coords_warmup_remaining(3);
+
+    // Simulate the decrement pattern used in send_session_data
+    for expected in (0..3).rev() {
+        assert!(entry.coords_warmup_remaining() > 0);
+        entry.set_coords_warmup_remaining(entry.coords_warmup_remaining() - 1);
+        assert_eq!(entry.coords_warmup_remaining(), expected);
+    }
+
+    assert_eq!(entry.coords_warmup_remaining(), 0,
+        "Counter should reach 0 after N decrements");
+}
+
+#[test]
+fn test_coords_warmup_config_default() {
+    let config = crate::config::Config::new();
+    assert_eq!(config.node.session.coords_warmup_packets, 5,
+        "Default coords_warmup_packets should be 5");
+}
+
 // ============================================================================
 // Unit tests: Identity cache
 // ============================================================================