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

Cargo.lock

@@ -4449,8 +4449,11 @@ name = "quicprochat-p2p"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "chacha20poly1305",
+ "ciborium",
  "hex",
+ "hkdf",
  "iroh",
  "quicprochat-core",
  "rand 0.8.5",
@@ -4458,8 +4461,10 @@ dependencies = [
  "serde_json",
  "sha2 0.10.9",
  "tempfile",
+ "thiserror 1.0.69",
  "tokio",
  "tracing",
+ "x25519-dalek",
  "zeroize",
 ]