Fix drag-handoff bugs: gate FX re-resolve on outside-panel, cache unresolvable OS-drag verdict, block double FX-add retry

src/shell/actions/drag_out_win.h

@@ -26,4 +26,13 @@ namespace reasampler {
 // (DROPEFFECT_COPY); the return is advisory — a failed drag surfaces no error.
 bool initiateDragOut(HWND__* panelHwnd, const std::vector<std::string>& absolutePaths);
 
+// Cheap, side-effect-free readiness probe: true iff initiateDragOut would actually be able to
+// START a drag for `absolutePaths` right now. Call this BEFORE tearing down internal drag
+// state (release capture, clear drag fields) — an OS that isn't ready (OLE unavailable, or the
+// path list can't build a CF_HDROP) must not consume the gesture the same way an empty payload
+// would. On Windows this re-runs the same OLE-init + HDROP-build checks initiateDragOut does,
+// freeing the probe HGLOBAL immediately; SWELL exposes no such probe, so macOS/Linux reduces to
+// the non-empty check alone.
+bool canInitiateDragOut(const std::vector<std::string>& absolutePaths);
+
 } // namespace reasampler