feat: Sprint 9 — mesh identity, store-and-forward, broadcast channels

Self-sovereign mesh networking for offline-capable Freifunk deployments.

- MeshIdentity: Ed25519 keypair-based identity without AS registration,
  JSON-persisted seed + known peers directory, sign/verify
- MeshEnvelope: signed store-and-forward envelope with TTL, hop_count,
  max_hops, SHA-256 dedup ID, Ed25519 signature verification
- MeshStore: in-memory message queue with dedup, per-recipient capacity
  limits, TTL-based garbage collection
- BroadcastChannel: symmetric ChaCha20-Poly1305 encrypted topic-based
  pub/sub for mesh announcements, no MLS overhead
- BroadcastManager: subscribe/unsubscribe/create channels by topic
- P2pNode integration: send_mesh(), receive_mesh(), forward_stored(),
  subscribe(), create_broadcast(), broadcast()
- Extended mesh REPL: /mesh send, /mesh broadcast, /mesh subscribe,
  /mesh route, /mesh identity, /mesh store (feature-gated)

28 P2P tests pass (21 existing + 7 broadcast). All builds clean.

Cargo.lock

@@ -5560,7 +5560,15 @@ name = "quicproquo-p2p"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "chacha20poly1305 0.10.1",
+ "hex",
  "iroh",
+ "quicproquo-core",
+ "rand 0.8.5",
+ "serde",
+ "serde_json",
+ "sha2 0.10.9",
+ "tempfile",
  "tokio",
  "tracing",
 ]