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

schemas/node.capnp

@@ -82,7 +82,10 @@ interface NodeService {
 
   # Create a 1:1 channel between the caller and the given peer. Returns a 16-byte channelId (UUID).
   # Both members can enqueue/fetch for this channel; recipientKey must be the other member.
-  createChannel @18 (peerKey :Data, auth :Auth) -> (channelId :Data);
+  # wasNew is true iff this call created the channel; false if it already existed.
+  # The caller who receives wasNew=true is the MLS group initiator and must send the Welcome.
+  # The caller who receives wasNew=false must wait for the peer's Welcome via the background poller.
+  createChannel @18 (peerKey :Data, auth :Auth) -> (channelId :Data, wasNew :Bool);
 
   # Resolve a username to its Ed25519 identity key (32 bytes).
   # Returns empty Data if the username is not registered.