S17 drop-and-load onto track FX button; S13 editor drop-accept (relay degraded)

S17: drag_out InstrumentDrop gesture + instrument_drop blob reusing the
instrument's own serializer; bank_panel FX hover-track + add-VST/vst_chunk
inject. S13 relay deferred (read-only bridge) — editor shows drop affordance.

src/drag_out.cpp

@@ -19,7 +19,13 @@ bool insideClient(int px, int py, const PanelClientRect& c) {
 DragGesture decideGesture(int px, int py, const PanelClientRect& client,
                           const DragState& state) {
     if (!state.dragging || !state.hasArmedSamples) return DragGesture::None;
-    return insideClient(px, py, client) ? DragGesture::Internal : DragGesture::OsDrag;
+    if (insideClient(px, py, client)) return DragGesture::Internal;
+    // Outside the client rect (M11 boundary), refined by S17: a SINGLE-capture drag that is
+    // still over REAPER's own UI is an instrument drop (heading for a track's FX button);
+    // anything else (a multi-capture payload, or the pointer off REAPER entirely) is the
+    // unchanged M11 OS drag-out.
+    if (state.singleCapture && state.overReaperUi) return DragGesture::InstrumentDrop;
+    return DragGesture::OsDrag;
 }
 
 PathList assemblePathList(const std::vector<ResolvedSample>& resolved) {