Fix LookupResponse proof verification

Include target_coords in proof_bytes() signed data to prevent transit
nodes from substituting fake coordinates. Add mandatory signature
verification at the originator using the target's public key from
identity_cache (guaranteed available since lookups are only initiated
from contexts where the key is already cached).

Verification failure discards the response. Identity cache miss (should
never happen) logs an error and discards. Add four new tests covering
verification success, failure, cache miss, and coordinate substitution
detection.

src/protocol/discovery.rs

@@ -200,11 +200,13 @@ impl LookupResponse {
 
     /// Get the bytes that should be signed as proof.
     ///
-    /// Format: request_id (8) || target (16)
-    pub fn proof_bytes(request_id: u64, target: &NodeAddr) -> Vec<u8> {
-        let mut bytes = Vec::with_capacity(24);
+    /// Format: request_id (8) || target (16) || coords_encoding (2 + 16×n)
+    pub fn proof_bytes(request_id: u64, target: &NodeAddr, target_coords: &TreeCoordinate) -> Vec<u8> {
+        let coord_size = 2 + target_coords.entries().len() * 16;
+        let mut bytes = Vec::with_capacity(24 + coord_size);
         bytes.extend_from_slice(&request_id.to_le_bytes());
         bytes.extend_from_slice(target.as_bytes());
+        encode_coords(target_coords, &mut bytes);
         bytes
     }
 
@@ -329,11 +331,17 @@ mod tests {
     #[test]
     fn test_lookup_response_proof_bytes() {
         let target = make_node_addr(42);
-        let bytes = LookupResponse::proof_bytes(12345, &target);
+        let coords = make_coords(&[42, 1, 0]);
+        let bytes = LookupResponse::proof_bytes(12345, &target, &coords);
 
-        assert_eq!(bytes.len(), 24); // 8 + 16
+        // 8 (request_id) + 16 (target) + 2 (count) + 3*16 (coords) = 74
+        assert_eq!(bytes.len(), 74);
         assert_eq!(&bytes[0..8], &12345u64.to_le_bytes());
         assert_eq!(&bytes[8..24], target.as_bytes());
+
+        // Verify coordinate encoding is present
+        let count = u16::from_le_bytes([bytes[24], bytes[25]]);
+        assert_eq!(count, 3); // 3 entries in coords
     }
 
     #[test]
@@ -372,7 +380,7 @@ mod tests {
         // Create a dummy signature for testing
         let secp = Secp256k1::new();
         let keypair = secp256k1::Keypair::new(&secp, &mut rand::thread_rng());
-        let proof_data = LookupResponse::proof_bytes(999, &target);
+        let proof_data = LookupResponse::proof_bytes(999, &target, &coords);
         use sha2::Digest;
         let digest: [u8; 32] = sha2::Sha256::digest(&proof_data).into();
         let sig = secp.sign_schnorr(&digest, &keypair);