fix(drop): recognize TCP/MCP FX-button hotspot for instrument drop (S-VIEW-BUG-1)

The FX-hotspot classifier matched only fx_ (the FX-chain/floating windows),
so a drop on a track TCP FX button (info tcp*) never armed the drop and the
capture fell through to arrange as audio. Widen to fx_/tcp/mcp in a pure,
unit-tested predicate; shell calls it.

src/instrument_drop_win.cpp

@@ -5,10 +5,10 @@
 
 #include "instrument_drop_win.h"
 
-#include <cstring>
 #include <string>
 
-#include "app_version.h"  // vstPluginName() — the CHANNEL-correct FX name (stable/beta pairing)
+#include "app_version.h"      // vstPluginName() — the CHANNEL-correct FX name (stable/beta pairing)
+#include "instrument_drop.h"  // infoNamesFxHotspot — the PURE, unit-tested hotspot classifier
 
 #include "reaper_plugin.h"
 
@@ -23,19 +23,6 @@
 
 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};
@@ -46,6 +33,8 @@ FxDropTarget resolveFxDropTarget(int screenX, int screenY) {
     MediaTrack* track = GetThingFromPoint(screenX, screenY, info, sizeof(info));
     out.track = track;
     out.overReaperUi = (track != nullptr) || (info[0] != '\0');
+    // S-VIEW-BUG-1: the hotspot is either the FX chain/floating window ("fx_*") OR the track/
+    // mixer panel that hosts the FX button ("tcp*"/"mcp*"). The pure classifier owns the rule.
     out.overFxHotspot = (track != nullptr) && infoNamesFxHotspot(info);
     return out;
 }