Wire impl classes to interfaces (inherit; relocate SendHandler)

- Waku : IKernel, MessagingClient : IMessagingClient,
  ReliableChannelManager : IReliableChannelManager.
- The operation procs already live in PR#3989's decomposed */api/ modules and
  stay as plain procs (nothing dispatches through the interface types, so no
  method-ization is needed).
- SendHandler now lives in reliable_channel_manager_api.nim (its PR#3975 home);
  removed the duplicate from reliable_channel.nim, which re-exports the
  interface module so channels/api/{channel_lifecycle,send} still see it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

logos_delivery/channels/reliable_channel_manager.nim

@@ -28,7 +28,7 @@ type
     ## channel API. Placeholder for now (segmentation / SDS / rate-limit defaults
     ## will move here in a follow-up PR); kept so each layer owns its own config.
 
-  ReliableChannelManager* = ref object
+  ReliableChannelManager* = ref object of IReliableChannelManager
     channels*: Table[ChannelId, ReliableChannel] ## read by `channels/api.nim`
     messagingClient: MessagingClient ## The channel layer chains onto messaging.
     sendHandler*: SendHandler