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/mod.rs

@@ -963,6 +963,13 @@ impl Node {
         }
     }
 
+    /// Check if a node's identity is in the cache (without LRU touch).
+    pub(crate) fn has_cached_identity(&self, addr: &NodeAddr) -> bool {
+        let mut prefix = [0u8; 15];
+        prefix.copy_from_slice(&addr.as_bytes()[0..15]);
+        self.identity_cache.contains_key(&prefix)
+    }
+
     /// Number of identity cache entries.
     pub fn identity_cache_len(&self) -> usize {
         self.identity_cache.len()