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

scripts/mesh-demo.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# Run the simulated LoRa + TCP relay integration example (no hardware).
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$ROOT"
+exec cargo run -p quicprochat-p2p --example mesh_lora_relay_demo