Fix identity cache miss during lookup response verification

Guard discovery triggers in handle_path_broken() and
handle_coords_required() with has_cached_identity() check. When the
XK responder receives an error signal before msg3 completes, the
initiator's identity is unknown, making LookupResponse proof
verification impossible. Skip discovery in this case — the handshake
retry mechanism handles recovery.

Downgrade the identity_cache miss log from ERROR to WARN since it's a
known race condition, not a bug.

src/node/handlers/discovery.rs

@@ -7,7 +7,7 @@
 use crate::node::{Node, RecentRequest};
 use crate::protocol::{LookupRequest, LookupResponse};
 use crate::{NodeAddr, PeerIdentity};
-use tracing::{debug, error, trace, warn};
+use tracing::{debug, trace, warn};
 
 impl Node {
     /// Handle an incoming LookupRequest from a peer.
@@ -153,10 +153,10 @@ impl Node {
             let target_pubkey = match self.lookup_by_fips_prefix(&prefix) {
                 Some((_addr, pubkey)) => pubkey,
                 None => {
-                    error!(
+                    warn!(
                         request_id = response.request_id,
                         target = %self.peer_display_name(&target),
-                        "identity_cache miss for lookup target — this is a bug"
+                        "identity_cache miss for lookup target, cannot verify proof"
                     );
                     return;
                 }

src/node/handlers/session.rs

@@ -731,7 +731,14 @@ impl Node {
                 "CoordsRequired response rate-limited, skipping standalone CoordsWarmup");
         }
 
-        self.maybe_initiate_lookup(&msg.dest_addr).await;
+        // Only trigger discovery if we have the target's identity cached —
+        // otherwise we can't verify the LookupResponse proof.
+        if self.has_cached_identity(&msg.dest_addr) {
+            self.maybe_initiate_lookup(&msg.dest_addr).await;
+        } else {
+            debug!(dest = %msg.dest_addr,
+                "Skipping discovery after CoordsRequired: no cached identity for target");
+        }
 
         // Reset coords warmup counter so the next N packets also include
         // COORDS_PRESENT, re-warming transit caches along the path.
@@ -783,8 +790,16 @@ impl Node {
         // Invalidate stale cached coordinates
         self.coord_cache.remove(&msg.dest_addr);
 
-        // Trigger re-discovery to get fresh coordinates
-        self.maybe_initiate_lookup(&msg.dest_addr).await;
+        // Trigger re-discovery to get fresh coordinates, but only if we have
+        // the target's identity cached — otherwise we can't verify the
+        // LookupResponse proof. This avoids a race when the XK responder
+        // receives PathBroken before msg3 completes (identity unknown).
+        if self.has_cached_identity(&msg.dest_addr) {
+            self.maybe_initiate_lookup(&msg.dest_addr).await;
+        } else {
+            debug!(dest = %msg.dest_addr,
+                "Skipping discovery after PathBroken: no cached identity for target");
+        }
 
         // Reset coords warmup counter so the next N packets include
         // COORDS_PRESENT, re-warming transit caches along the new path.