[client] clipboard: add transport provider interface

Route clipboard exchange through a provider interface. Connected
transports take priority while SPICE remains the fallback.

Track asynchronous X11 and Wayland transfers by request, preserving
clipboard ownership across provider changes.

Use PureSpice clipboard status notifications for capability and
lifecycle updates.

client/include/interface/transport.h

@@ -28,6 +28,7 @@
 #include "common/framebuffer.h"
 #include "common/types.h"
 #include "interface/audio.h"
+#include "interface/clipboard.h"
 #include "interface/input.h"
 
 #define LG_TRANSPORT_MAX_DAMAGE_RECTS LG_MAX_FRAME_DAMAGE_RECTS
@@ -255,6 +256,11 @@ typedef struct LG_TransportOps
    * valid until disconnect; NULL indicates that this session has no audio. */
   const LG_AudioOps *(*getAudioOps)(LG_Transport * transport,
       void ** opaque);
+  /* Queried after connect. The returned operations and opaque value remain
+   * valid until disconnect; NULL indicates that this session has no
+   * clipboard. */
+  const LG_ClipboardOps *(*getClipboardOps)(LG_Transport * transport,
+      void ** opaque);
   bool (*attachRenderer)(LG_Transport * transport,
       const LG_RendererInterop * interop);
   void (*detachRenderer)(LG_Transport * transport);