arch: eliminate last _x11_raw escape from src/ (winspector)

The Window Inspector's 'select window' feature grabbed the pointer on
the root window then read XEvent.xbutton.subwindow to learn which
window was clicked.  This was the last access to _x11_raw in shared
src/ code.

Add a subwindow field to WMEButton.  The X11 backend's WMEvent
translation populates it from xbutton.subwindow.  The Wayland backend
leaves it 0 (Wayland has no equivalent grab semantic — the click
reports the actual frame as event.window directly).  winspector.c
now uses event.u.button.subwindow with fallback to
event.u.button.window, working on both backends.

Net: src/ has zero _x11_raw escapes from shared code.  Remaining
references are inside the X11 backend (legitimately consuming the
field set by its own event translator).

WINGs/WINGs/WMEvent.h

@@ -82,6 +82,7 @@ typedef struct {
 
 typedef struct {
     WNativeWindow window;
+    WNativeWindow subwindow;    /* deepest child under pointer at click time */
     unsigned int   button;      /* 1=left, 2=middle, 3=right, 4+=extra */
     unsigned int   state;       /* modifier mask */
     int            x, y;

WINGs/wevent_x11.c

@@ -40,6 +40,7 @@ xev_to_wme(const XEvent *xev, WMEvent *out)
     case ButtonRelease:
         out->type             = (xev->type == ButtonPress) ? WME_BUTTON_PRESS : WME_BUTTON_RELEASE;
         out->u.button.window  = (WNativeWindow)xev->xbutton.window;
+        out->u.button.subwindow = (WNativeWindow)xev->xbutton.subwindow;
         out->u.button.button  = xev->xbutton.button;
         out->u.button.state   = xev->xbutton.state;
         out->u.button.x       = xev->xbutton.x;

src/winspector.c

@@ -1035,8 +1035,13 @@ static void selectWindow(WMWidget *bPtr, void *data)
 	wm_backend->pointer_ungrab(0);
 
 	{
-		XEvent *_xev = (XEvent *)event._x11_raw;
-		iwin = _xev ? wWindowFor(_xev->xbutton.subwindow) : NULL;
+		/* On X11 with the pointer grabbed on root_win, the click reports
+		 * window=root and subwindow=clicked window.  On Wayland there
+		 * is no such grab semantic; the event reports window=clicked
+		 * window directly with subwindow==0.  Prefer subwindow when set. */
+		WNativeWindow target = event.u.button.subwindow ?
+			event.u.button.subwindow : event.u.button.window;
+		iwin = wWindowFor(target);
 	}
 	if (iwin && !iwin->flags.internal_window && iwin != wwin && !iwin->flags.inspector_open) {
 		iwin->flags.inspector_open = 1;