feat: DM epoch fix, federation relay, and mDNS mesh discovery

- schema: createChannel returns wasNew :Bool to elect the MLS initiator
  unambiguously; prevents duplicate group creation on concurrent /dm calls
- core: group helpers for epoch tracking and key-package lifecycle
- server: federation subsystem — mTLS QUIC server-to-server relay with
  Cap'n Proto RPC; enqueue/batchEnqueue relay unknown recipients to their
  home domain via FederationClient
- server: mDNS _quicproquo._udp.local. service announcement on startup
- server: storage + sql_store — identity_exists, peek/ack, federation
  home-server lookup helpers
- client: /mesh peers REPL command (mDNS discovery, feature = "mesh")
- client: MeshDiscovery — background mDNS browse with ServiceDaemon
- client: was_new=false path in cmd_dm waits for peer Welcome instead of
  creating a duplicate initiator group
- p2p: fix ALPN from quicnprotochat/p2p/1 → quicproquo/p2p/1
- workspace: re-include quicproquo-p2p in members

crates/quicproquo-client/Cargo.toml

@@ -59,6 +59,17 @@ hex           = { workspace = true }
 # Secure password prompting (no echo)
 rpassword     = "5"
 
+# mDNS discovery for mesh mode (Freifunk). Only compiled with --features mesh.
+mdns-sd = { version = "0.12", optional = true }
+
+# Optional P2P transport for direct node-to-node messaging.
+quicproquo-p2p = { path = "../quicproquo-p2p", optional = true }
+
+[features]
+# Enable mesh-mode features: mDNS local peer discovery + P2P transport.
+# Build: cargo build -p quicproquo-client --features mesh
+mesh = ["dep:mdns-sd", "dep:quicproquo-p2p"]
+
 [dev-dependencies]
 dashmap       = { workspace = true }
 assert_cmd    = "2"