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.h

@@ -47,10 +47,11 @@ FxDropTarget resolveFxDropTarget(int screenX, int screenY);
 // (the instrument_drop::buildInstrumentDropChunk output) as its component state so it plays
 // the dragged capture. `chunkBase64` is the base64 vst_chunk. Wraps the add + inject in one
 // REAPER undo block (mirrors the bank-verb undo discipline). Returns true on success (the FX
-// was added and the chunk written), false on any failure (add returned -1, or the chunk write
-// was rejected). A false return leaves at most the added FX (no partial-state confusion — the
-// caller surfaces nothing; a failed add is visible by nothing happening). NEVER inserts a
-// timeline item; the ONLY mutations are the FX instance + its state, both undoable.
+// was added and the chunk written), false on any failure. All-or-nothing: if the chunk write
+// fails after a successful add, the freshly-added FX instance is removed via TrackFX_Delete
+// before returning false, leaving the track exactly as it was (no orphaned empty-state FX).
+// NEVER inserts a timeline item; the ONLY mutations are the FX instance + its state, both
+// undoable.
 bool performInstrumentDrop(MediaTrack* track, const std::string& chunkBase64);
 
 }  // namespace reasampler