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/instrument_drop_win.cpp

@@ -0,0 +1,83 @@
+// instrument_drop_win — the REAPER shell for S17 drop-and-load. See instrument_drop_win.h.
+//
+// Compiled into the reaper_reasampler MODULE. Includes reaper_plugin_functions.h WITHOUT
+// REAPERAPI_IMPLEMENT (main.cpp owns the pointers; here they are extern via the WANT list).
+
+#include "instrument_drop_win.h"
+
+#include <cstring>
+#include <string>
+
+#include "app_version.h"  // vstPluginName() — the CHANNEL-correct FX name (stable/beta pairing)
+
+#include "reaper_plugin.h"
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_GetThingFromPoint
+#define REAPERAPI_WANT_TrackFX_AddByName
+#define REAPERAPI_WANT_TrackFX_SetNamedConfigParm
+#define REAPERAPI_WANT_Undo_BeginBlock2
+#define REAPERAPI_WANT_Undo_EndBlock2
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+namespace {
+
+// GetThingFromPoint's info string prefixes (verified against reaper_plugin_functions.h:
+// "Updates infoOut with information such as 'arrange', 'fx_chain', 'fx_0' ... If a track
+// panel is hit, string will begin with 'tcp' or 'mcp' or 'tcp.mute' etc"). The FX region
+// reports "fx_chain" (the FX list area) or "fx_N" (a specific FX button). We treat either
+// as the FX hotspot — the S17 drop target.
+bool infoNamesFxHotspot(const char* info) {
+    return std::strncmp(info, "fx_", 3) == 0;
+}
+
+}  // namespace
+
+FxDropTarget resolveFxDropTarget(int screenX, int screenY) {
+    FxDropTarget out;
+    char info[256] = {0};
+    // GetThingFromPoint returns the track under the point (may be null for a non-track thing)
+    // and fills `info` with what was hit. A non-empty info OR a non-null track means the point
+    // is over REAPER's own UI; a null track with an empty info means the pointer has left
+    // REAPER entirely (over another app / the desktop) — the OsDrag boundary.
+    MediaTrack* track = GetThingFromPoint(screenX, screenY, info, sizeof(info));
+    out.track = track;
+    out.overReaperUi = (track != nullptr) || (info[0] != '\0');
+    out.overFxHotspot = (track != nullptr) && infoNamesFxHotspot(info);
+    return out;
+}
+
+bool performInstrumentDrop(MediaTrack* track, const std::string& chunkBase64) {
+    if (!track || chunkBase64.empty()) return false;
+
+    // The CHANNEL-correct FX name: "VST3:ReaSampler 9000" on stable, "VST3:ReaSampler 9000
+    // beta" on beta. Sourcing it from app_version::vstPluginName() (the same accessor the VST
+    // factory display name derives from) keeps the pairing invariant intact — a beta extension
+    // drops the beta VST, a stable extension the stable VST — with no literal to drift.
+    const std::string fxName = "VST3:" + vstPluginName();
+
+    // One undo point for the whole gesture (mirrors the bank-verb undo discipline). Both the
+    // FX add and the state write are REAPER-undoable, so Ctrl-Z removes the instance cleanly.
+    Undo_BeginBlock2(nullptr);
+
+    // Negative `instantiate` => always create a NEW instance (verified in the header). recFX
+    // = false: a normal track FX chain instance, not a record/monitoring FX.
+    const int fxIndex = TrackFX_AddByName(track, fxName.c_str(), /*recFX=*/false,
+                                          /*instantiate=*/-1);
+    bool ok = false;
+    if (fxIndex >= 0) {
+        // Inject the instrument's OWN component-state blob (the dragged capture pre-selected)
+        // via the documented vst_chunk write-parm. The blob was built by the shared writer
+        // (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());
+    }
+
+    // The undo label reflects the placement-of-the-player framing (not a capture, not an insert).
+    Undo_EndBlock2(nullptr, "ReaSampler: drop capture onto FX chain", -1);
+    return ok;
+}
+
+}  // namespace reasampler