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/src/client/rpc.rs

@@ -645,12 +645,16 @@ pub async fn resolve_identity(
     }
 }
 
-/// Create a 1:1 DM channel with a peer. Returns the 16-byte channel ID.
-/// If a channel already exists between the two users, returns the existing ID.
+/// Create a 1:1 DM channel with a peer.
+///
+/// Returns `(channel_id, was_new)` where `channel_id` is the stable 16-byte identifier and
+/// `was_new` is `true` iff this call created the channel for the first time. When `was_new` is
+/// `false`, the channel already existed (created by the peer), and the caller should wait for
+/// the peer's MLS Welcome to arrive via the background poller rather than creating a new MLS group.
 pub async fn create_channel(
     client: &node_service::Client,
     peer_key: &[u8],
-) -> anyhow::Result<Vec<u8>> {
+) -> anyhow::Result<(Vec<u8>, bool)> {
     let mut req = client.create_channel_request();
     {
         let mut p = req.get();
@@ -665,14 +669,14 @@ pub async fn create_channel(
         .await
         .context("create_channel RPC failed")?;
 
-    let channel_id = resp
-        .get()
-        .context("create_channel: bad response")?
+    let reader = resp.get().context("create_channel: bad response")?;
+    let channel_id = reader
         .get_channel_id()
         .context("create_channel: missing channel_id")?
         .to_vec();
+    let was_new = reader.get_was_new();
 
-    Ok(channel_id)
+    Ok((channel_id, was_new))
 }
 
 /// Return the current Unix timestamp in milliseconds.