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/Cargo.toml

@@ -14,7 +14,8 @@ workspace = true
 
 [dependencies]
 iroh        = "0.96"
-tokio       = { version = "1", features = ["macros", "rt-multi-thread", "time", "sync"] }
+tokio       = { version = "1", features = ["macros", "rt-multi-thread", "time", "sync", "net", "io-util"] }
+async-trait = "0.1"
 tracing     = "0.1"
 anyhow      = "1"
 
@@ -22,6 +23,7 @@ anyhow      = "1"
 quicprochat-core = { path = "../quicprochat-core", default-features = false }
 serde           = { workspace = true }
 serde_json      = { workspace = true }
+ciborium        = { workspace = true }
 sha2            = { workspace = true }
 hex             = { workspace = true }
 
@@ -30,5 +32,10 @@ chacha20poly1305 = { workspace = true }
 rand             = { workspace = true }
 zeroize          = { workspace = true }
 
+# Lightweight mesh link handshake (X25519 ECDH + HKDF)
+x25519-dalek     = { workspace = true }
+hkdf             = { workspace = true }
+thiserror        = { workspace = true }
+
 [dev-dependencies]
 tempfile = "3"