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-p2p/src/lib.rs

@@ -15,8 +15,9 @@
 use iroh::{Endpoint, EndpointAddr, PublicKey, SecretKey};
 
 /// ALPN protocol identifier for quicproquo P2P messaging.
-/// Frozen at the original project name for wire compatibility.
-const P2P_ALPN: &[u8] = b"quicnprotochat/p2p/1";
+/// Updated from the original project name "quicnprotochat" to "quicproquo" (breaking wire change;
+/// all peers must be on the same version to connect).
+const P2P_ALPN: &[u8] = b"quicproquo/p2p/1";
 
 /// A P2P node backed by an iroh endpoint.
 ///