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.

crates/quicproquo-p2p/Cargo.toml

@@ -13,3 +13,17 @@ iroh        = "0.96"
 tokio       = { version = "1", features = ["macros", "rt-multi-thread", "time", "sync"] }
 tracing     = "0.1"
 anyhow      = "1"
+
+# Mesh identity & store-and-forward
+quicproquo-core = { path = "../quicproquo-core", default-features = false }
+serde           = { workspace = true }
+serde_json      = { workspace = true }
+sha2            = { workspace = true }
+hex             = { workspace = true }
+
+# Broadcast channels (ChaCha20-Poly1305 symmetric encryption)
+chacha20poly1305 = { workspace = true }
+rand             = { workspace = true }
+
+[dev-dependencies]
+tempfile = "3"