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

Cargo.toml

@@ -7,11 +7,11 @@ members = [
     "crates/quicproquo-client",
     "crates/quicproquo-gui",
     "crates/quicproquo-mobile",
+    # P2P crate uses iroh (~90 extra deps). Kept in the workspace so it can be
+    # referenced as an optional dependency; only compiled when the `mesh` feature
+    # is enabled on quicproquo-client.
+    "crates/quicproquo-p2p",
 ]
-# P2P-Crate (iroh-Transport) ist vom Default-Build ausgeschlossen,
-# um ~90 exklusive iroh-Dependencies nicht mitzukompilieren.
-# Quellcode bleibt im Repo für spätere Integration.
-exclude = ["crates/quicproquo-p2p"]
 
 # Shared dependency versions — bump here to affect the whole workspace.
 [workspace.dependencies]