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/examples/mesh_lora_relay_demo.rs

@@ -0,0 +1,96 @@
+//! Simulated mesh leg: **A (LoRa)** → **B (LoRa + TCP relay)** → **C (TCP)** → zurück über B → **A**.
+//!
+//! Uses [`quicprochat_p2p::transport_lora::LoRaMockMedium`] — keine Hardware.
+//!
+//! ```text
+//!   Node A                Node B                      Node C
+//!  LoRa addr 0x01   LoRa 0x02 + TCP listen      TCP (WiFi / LAN)
+//!       │                 │                           │
+//!       └──── LoRa ───────┘                           │
+//!                         └──────── TCP ──────────────┘
+//! ```
+//!
+//! Run: `cargo run -p quicprochat-p2p --example mesh_lora_relay_demo`
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use quicprochat_p2p::transport::{MeshTransport, TransportAddr};
+use quicprochat_p2p::transport_lora::{DutyCycleTracker, LoRaConfig, LoRaMockMedium};
+use quicprochat_p2p::transport_tcp::TcpTransport;
+
+const ADDR_A: [u8; 4] = [0x01, 0, 0, 0];
+const ADDR_B: [u8; 4] = [0x02, 0, 0, 0];
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    let medium = LoRaMockMedium::new();
+    let duty = Arc::new(DutyCycleTracker::new(3_600_000));
+
+    let lora_a = medium
+        .connect(ADDR_A, LoRaConfig::default(), Arc::clone(&duty))
+        .await?;
+    let lora_b = medium
+        .connect(ADDR_B, LoRaConfig::default(), Arc::clone(&duty))
+        .await?;
+
+    let tcp_b = TcpTransport::bind("127.0.0.1:0").await?;
+    let tcp_c = TcpTransport::bind("127.0.0.1:0").await?;
+
+    let c_listen = tcp_c.local_addr();
+    let b_listen = tcp_b.local_addr();
+    let c_addr = TransportAddr::Socket(c_listen);
+    let b_addr = TransportAddr::Socket(b_listen);
+
+    println!(
+        "LoRa mock mesh demo: B relays LoRa <-> TCP (B TCP {}, C TCP {})",
+        b_listen, c_listen
+    );
+
+    let relay = tokio::spawn(async move {
+        for _ in 0..2 {
+            tokio::select! {
+                p = lora_b.recv() => {
+                    let p = p.expect("B LoRa recv");
+                    println!("B: LoRa from {} -> TCP ({} bytes)", p.from, p.data.len());
+                    tcp_b.send(&c_addr, &p.data).await.expect("B TCP send to C");
+                }
+                p = tcp_b.recv() => {
+                    let p = p.expect("B TCP recv");
+                    println!("B: TCP -> LoRa A ({} bytes)", p.data.len());
+                    lora_b
+                        .send(&TransportAddr::LoRa(ADDR_A), &p.data)
+                        .await
+                        .expect("B LoRa send to A");
+                }
+            }
+        }
+    });
+
+    let c_task = tokio::spawn(async move {
+        let pkt = tcp_c.recv().await.expect("C TCP recv");
+        println!("C: got {} bytes from B relay", pkt.data.len());
+        assert_eq!(pkt.data, b"hello via mesh");
+        tcp_c
+            .send(&b_addr, b"ack from C")
+            .await
+            .expect("C TCP send");
+    });
+
+    tokio::time::sleep(Duration::from_millis(50)).await;
+
+    lora_a
+        .send(&TransportAddr::LoRa(ADDR_B), b"hello via mesh")
+        .await?;
+
+    let reply = lora_a.recv().await?;
+    println!("A: LoRa reply {} bytes", reply.data.len());
+    assert_eq!(reply.data, b"ack from C");
+
+    c_task.await.expect("node C task panicked");
+    relay.await.expect("relay task panicked");
+
+    lora_a.close().await.ok();
+    println!("Done: LoRa + TCP relay path OK.");
+    Ok(())
+}