Add channel-aware delivery and update roadmap

schemas/delivery.capnp

@@ -20,16 +20,21 @@ interface DeliveryService {
   # recipientKey : Ed25519 public key of the intended recipient (exactly 32 bytes).
   # payload      : Opaque byte string — a TLS-encoded MlsMessageOut blob or any
   #                other framed data the application layer wants to deliver.
+  # channelId    : Optional channel identifier (empty for legacy). A 16-byte UUID
+  #                is recommended for 1:1 channels.
+  # version      : Schema/wire version. Must be 0 (legacy) or 1 (this spec).
   #
   # The payload is appended to the recipient's FIFO queue. Returns immediately;
   # the recipient retrieves it via `fetch`.
-  enqueue @0 (recipientKey :Data, payload :Data) -> ();
+  enqueue @0 (recipientKey :Data, payload :Data, channelId :Data, version :UInt16) -> ();
 
   # Fetch and atomically drain all queued payloads for a given recipient.
   #
   # recipientKey : Ed25519 public key of the caller (exactly 32 bytes).
+  # channelId    : Optional channel identifier (empty for legacy).
+  # version      : Schema/wire version. Must be 0 (legacy) or 1 (this spec).
   #
   # Returns the complete queue in FIFO order and clears it. Returns an empty
   # list if there are no pending messages.
-  fetch @1 (recipientKey :Data) -> (payloads :List(Data));
+  fetch @1 (recipientKey :Data, channelId :Data, version :UInt16) -> (payloads :List(Data));
 }