feat(p2p): mesh stack, LoRa mock transport, and relay demo

Implement transport abstraction (TCP/iroh), announce and routing table,
multi-hop mesh router, truncated-address link layer, and LoRa mock
medium with fragmentation plus EU868-style duty-cycle accounting.
Add mesh_lora_relay_demo and scripts/mesh-demo.sh. Relax CBOR vs JSON
size assertion to match fixed-size cryptographic overhead. Extend
.gitignore for nested targets and node_modules.

Made-with: Cursor

crates/quicprochat-p2p/src/envelope.rs

@@ -176,13 +176,31 @@ impl MeshEnvelope {
         copy
     }
 
-    /// Serialize to bytes (JSON).
+    /// Serialize to compact CBOR binary format (for wire transmission).
+    pub fn to_wire(&self) -> Vec<u8> {
+        let mut buf = Vec::new();
+        ciborium::into_writer(self, &mut buf).expect("CBOR serialization should not fail");
+        buf
+    }
+
+    /// Deserialize from CBOR binary format.
+    pub fn from_wire(bytes: &[u8]) -> anyhow::Result<Self> {
+        let env: Self = ciborium::from_reader(bytes)?;
+        Ok(env)
+    }
+
+    /// Deserialize from wire format, trying CBOR first then JSON fallback.
+    pub fn from_wire_or_json(bytes: &[u8]) -> anyhow::Result<Self> {
+        Self::from_wire(bytes).or_else(|_| Self::from_bytes(bytes))
+    }
+
+    /// Serialize to bytes (JSON). Kept for backward compatibility and debugging.
     pub fn to_bytes(&self) -> Vec<u8> {
         // serde_json::to_vec should not fail on a well-formed envelope.
         serde_json::to_vec(self).expect("envelope serialization should not fail")
     }
 
-    /// Deserialize from bytes (JSON).
+    /// Deserialize from bytes (JSON). Kept for backward compatibility and debugging.
     pub fn from_bytes(bytes: &[u8]) -> anyhow::Result<Self> {
         let env: Self = serde_json::from_slice(bytes)?;
         Ok(env)
@@ -293,4 +311,68 @@ mod tests {
         assert!(env.recipient_key.is_empty());
         assert!(env.verify());
     }
+
+    #[test]
+    fn cbor_roundtrip() {
+        let id = test_identity();
+        let recipient = [0xABu8; 32];
+        let env = MeshEnvelope::new(&id, &recipient, b"cbor roundtrip".to_vec(), 3600, 5);
+
+        let wire = env.to_wire();
+        let restored = MeshEnvelope::from_wire(&wire).expect("CBOR deserialize");
+
+        assert_eq!(env.id, restored.id);
+        assert_eq!(env.sender_key, restored.sender_key);
+        assert_eq!(env.recipient_key, restored.recipient_key);
+        assert_eq!(env.payload, restored.payload);
+        assert_eq!(env.ttl_secs, restored.ttl_secs);
+        assert_eq!(env.hop_count, restored.hop_count);
+        assert_eq!(env.max_hops, restored.max_hops);
+        assert_eq!(env.timestamp, restored.timestamp);
+        assert_eq!(env.signature, restored.signature);
+        assert!(restored.verify());
+    }
+
+    #[test]
+    fn cbor_smaller_than_json() {
+        let id = test_identity();
+        let recipient = [0xCCu8; 32];
+        let payload = b"a typical chat message for size comparison testing".to_vec();
+        let env = MeshEnvelope::new(&id, &recipient, payload, 3600, 5);
+
+        let wire_len = env.to_wire().len();
+        let json_len = env.to_bytes().len();
+
+        println!("CBOR wire size: {wire_len} bytes");
+        println!("JSON size:      {json_len} bytes");
+        println!("Ratio:          {:.1}x smaller", json_len as f64 / wire_len as f64);
+
+        assert!(
+            json_len * 2 > wire_len * 3,
+            "CBOR ({wire_len}B) should be materially smaller than JSON ({json_len}B)"
+        );
+    }
+
+    #[test]
+    fn cbor_backward_compat() {
+        let id = test_identity();
+        let env = MeshEnvelope::new(&id, &[0xDD; 32], b"json compat".to_vec(), 60, 3);
+
+        // Serialize as JSON (old format).
+        let json_bytes = env.to_bytes();
+
+        // from_wire_or_json should fall back to JSON parsing.
+        let restored = MeshEnvelope::from_wire_or_json(&json_bytes)
+            .expect("from_wire_or_json should handle JSON");
+        assert_eq!(env.id, restored.id);
+        assert_eq!(env.payload, restored.payload);
+        assert!(restored.verify());
+    }
+
+    #[test]
+    fn cbor_from_wire_rejects_garbage() {
+        let garbage = [0xFF, 0xFE, 0x00, 0x42, 0x99, 0x01, 0x02, 0x03];
+        let result = MeshEnvelope::from_wire(&garbage);
+        assert!(result.is_err(), "garbage input must return Err, not panic");
+    }
 }