HUD: inventory drag & drop, UI hit tests, centred overlays

- Mouse move events carry previous and current x (+4, +0xc); button events
  x at +8. All are shifted, and restored to the saved values.
- The cursor object's previous x (+0xc) is shifted too: picking up an item
  writes the cursor position into the event (0x6559a0).
- GetMouseClientPos returns the shifted position inside a moved window.
- Window hit tests (vf1c 0x731d40, taskbar 0x6dfb10) called outside a window
  scope, e.g. 'is UI under the cursor' for world picking (0x75a370), use the
  offset the top-level window was last drawn with.
- The item carried by the cursor (0x6555e0) and the UI manager's centred
  status line / message box (0x75bbf0, 0x75c200) draw with the centre offset.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/mods/hud.h

@@ -5,4 +5,8 @@ namespace mods::hud {
 // Installs the Direct3D device hooks the HUD layout needs (once per device).
 void OnDevice(void* device);
 
+// Horizontal offset of the moved window whose render or event method runs on
+// the current thread (0 outside): polled mouse positions are reduced by it.
+int CursorOffsetX();
+
 } // namespace mods::hud