fix(instrument_drop_win): make performInstrumentDrop all-or-nothing

On chunk-write failure, call TrackFX_Delete(track, fxIndex) to remove
the just-added empty FX instance before returning false. Signature
verified at reaper_plugin_functions.h:7236. Update the header comment
from "leaves at most the added FX" to the new all-or-nothing contract.

src/instrument_drop_win.cpp

@@ -15,6 +15,7 @@
 #define REAPERAPI_MINIMAL
 #define REAPERAPI_WANT_GetThingFromPoint
 #define REAPERAPI_WANT_TrackFX_AddByName
+#define REAPERAPI_WANT_TrackFX_Delete
 #define REAPERAPI_WANT_TrackFX_SetNamedConfigParm
 #define REAPERAPI_WANT_Undo_BeginBlock2
 #define REAPERAPI_WANT_Undo_EndBlock2
@@ -73,6 +74,12 @@ bool performInstrumentDrop(MediaTrack* track, const std::string& chunkBase64) {
         // (instrument_drop::buildInstrumentDropChunk -> sample_map::serializeComponentState),
         // so these bytes are exactly what ReaSampler 9000's setState accepts.
         ok = TrackFX_SetNamedConfigParm(track, fxIndex, "vst_chunk", chunkBase64.c_str());
+        if (!ok) {
+            // All-or-nothing: if the chunk write fails, remove the empty FX instance we just
+            // added so the track is left exactly as it was. TrackFX_Delete signature (verified
+            // in reaper_plugin_functions.h:7236): bool TrackFX_Delete(MediaTrack*, int fx).
+            TrackFX_Delete(track, fxIndex);
+        }
     }
 
     // The undo label reflects the placement-of-the-player framing (not a capture, not an insert).