WINGs: implement Wayland selection fetch (middle-click paste)

Add selection_fetch vtable slot for synchronous selection retrieval.
On Wayland, WMRequestSelection uses this to fetch clipboard/primary
selection data directly (via W_ClipboardFetchFn -> seat pipe read)
and calls the callback immediately. On X11, the async path
(selection_request -> SelectionNotify event) is unchanged.

This makes middle-click paste work in WINGs text widgets on Wayland.

WINGs/WINGs/WINGsP.h.in

@@ -621,6 +621,7 @@ typedef struct W_ViewBackend {
     Bool  (*selection_acquire)(void *vctx, struct W_View *view, Atom selection, Time timestamp);
     void  (*selection_release)(void *vctx, struct W_View *view, Atom selection);
     Bool  (*selection_request)(void *vctx, struct W_View *view, Atom selection, Atom target, Time timestamp);
+    char *(*selection_fetch)(void *vctx, Atom selection, int *len_out);
 
     /* Platform-specific post-init (e.g. X11 atoms, IM setup) */
     void  (*platform_init)(struct W_Screen *scr);

WINGs/selection.c

@@ -362,6 +362,20 @@ WMRequestSelection(WMView * view, Atom selection, Atom target, Time timestamp,
 	SelectionCallback *handler;
 
 	const W_ViewBackend *vb = view->screen->viewBackend;
+
+	/* Synchronous path (Wayland): fetch data directly and call callback */
+	if (vb->selection_fetch) {
+		int len = 0;
+		char *text = vb->selection_fetch(view->screen->viewBackendCtx, selection, &len);
+		if (!text)
+			return False;
+		WMData *data = WMCreateDataWithBytesNoCopy(text, len, wfree);
+		(*callback)(view, selection, target, timestamp, cdata, data);
+		WMReleaseData(data);
+		return True;
+	}
+
+	/* Async path (X11): initiate conversion, callback fires on SelectionNotify */
 	if (!vb->selection_request ||
 	    !vb->selection_request(view->screen->viewBackendCtx, view, selection, target, timestamp))
 		return False;

WINGs/wview_wl.c

@@ -483,6 +483,17 @@ static void wl_selection_release(void *vctx, W_View *view, Atom selection)
 	(void)vctx; (void)view; (void)selection;
 }
 
+static char *wl_selection_fetch(void *vctx, Atom selection, int *len_out)
+{
+	(void)vctx; (void)selection;
+	/* Use the clipboard fetch path (reads from seat selection via pipe) */
+	if (W_ClipboardFetchFn)
+		return W_ClipboardFetchFn(len_out);
+	if (len_out)
+		*len_out = 0;
+	return NULL;
+}
+
 static char *wl_clipboard_buf = NULL;
 static int wl_clipboard_len = 0;
 
@@ -605,6 +616,7 @@ const W_ViewBackend W_ViewBackendWL = {
 	.selection_acquire    = wl_selection_acquire,
 	.selection_release    = wl_selection_release,
 	.selection_request    = NULL,
+	.selection_fetch      = wl_selection_fetch,
 	.platform_init        = NULL,
 	.flush                = wl_flush,
 	.clipboard_store      = wl_clipboard_store,

WINGs/wview_x11.c

@@ -564,6 +564,7 @@ const W_ViewBackend W_ViewBackendX11 = {
 	.selection_acquire    = x11_selection_acquire,
 	.selection_release    = x11_selection_release,
 	.selection_request    = x11_selection_request,
+	.selection_fetch      = NULL,
 	.platform_init        = W_X11PlatformInit,
 	.flush                = x11_flush,
 	.clipboard_store      = x11_clipboard_store,