Reshape per-layer API into api/ folders and thin the FFI over them

Each layer now separates its constructible core from its public surface:

  - core module (waku.nim / messaging_client.nim /
    reliable_channel_manager.nim): the type plus new/start/stop and the
    private construction helpers.
  - api/ folder: one module per differentiated set of operations
    (waku: topics/relay/filter/lightpush/store/peer_manager/discovery/
    debug/health) plus an events surface.

The waku api is reshaped to be the complete operation surface the C
bindings need, so the library no longer reaches into node internals:
relayPublish returns the message hash, relaySubscribe takes an optional
handler, filter/lightpush auto-select the service peer, connectedPeersInfo
returns structured data, pingPeer honours the timeout, plus
relayNumPeersInMesh / relayNumConnectedPeers / isOnline. library/ is now a
thin C-ABI shim: each {.ffi.} proc only marshals cstring/JSON/callbacks and
delegates to ctx.myLib[].waku.<op> (or messagingClient.<op>).
app_callbacks re-exports the modules defining its handler types, which the
included FFI files previously relied on by leakage.

Events move next to the surface that owns them, with each dependency kept
pointing the right way:

  - waku/events/ relocated under waku/api/events/.
  - channel events live in channels/api/events.nim.
  - the four messaging-level message events move to messaging/api/events;
    MessageSeenEvent stays in waku because it is emitted by waku core, so
    moving it would make waku depend on the messaging layer.
  - delivery_events renamed to filter_subscribe_events to match the
    OnFilterSubscribe/Unsubscribe events it actually declares.

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 of IReliableChannelManager
+  ReliableChannelManager* = ref object
     channels*: Table[ChannelId, ReliableChannel] ## read by `channels/api.nim`
     messagingClient: MessagingClient ## The channel layer chains onto messaging.
     sendHandler*: SendHandler

logos_delivery/logos_delivery.nim

@@ -11,9 +11,6 @@
 
 import results, chronos, chronicles
 
-import logos_delivery/api/logos_delivery_api
-export logos_delivery_api
-
 # Each layer has a core module (type + new/start/stop) and an api/ folder whose
 # modules each implement a differentiated set of operations, plus an events
 # surface. The concentrator re-exports them so library consumers get the full

logos_delivery/waku/api/events/message_events.nim

@@ -0,0 +1,10 @@
+import brokers/event_broker
+import logos_delivery/api/types
+import logos_delivery/waku/[waku_core/message, waku_core/topics]
+export event_broker, types
+
+EventBroker:
+  # Internal event emitted when a message arrives from the network via any protocol
+  type MessageSeenEvent* = object
+    topic*: PubsubTopic
+    message*: WakuMessage