rekey: demote exhausted-budget rekey-abort logs from warn to debug

When an FMP msg1 or FSP msg3 rekey retransmission budget is exhausted, the
cycle is abandoned and retried on the next timer. On lossy or high-latency
links this is an expected, self-limiting outcome: the existing session stays
valid and keeps carrying traffic, so the abort is not a failure that warrants
operator-level visibility. Demote both abandon-cycle messages from warn to
debug to cut steady-state log noise on nodes with many flapping peers.

src/node/handlers/rekey.rs

@@ -260,7 +260,7 @@ impl Node {
             if let Some(peer) = self.peers.get_mut(&node_addr) {
                 peer.abandon_rekey();
             }
-            warn!(
+            debug!(
                 peer = %self.peer_display_name(&node_addr),
                 "FMP rekey aborted: msg1 unconfirmed after max retransmissions, abandoning cycle"
             );
@@ -356,7 +356,7 @@ impl Node {
             if let Some(entry) = self.sessions.get_mut(&node_addr) {
                 entry.abandon_rekey();
             }
-            warn!(
+            debug!(
                 peer = %self.peer_display_name(&node_addr),
                 "FSP rekey aborted: msg3 unconfirmed after max retransmissions, abandoning cycle"
             );