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/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;
+    }
 }