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.

tests/test_instrument_drop.cpp

@@ -105,6 +105,44 @@ static void testBase64DecodeRejectsMalformed() {
     CHECK(decodeBase64("Zg==Zg==").empty()); // interior padding (pad before the final quad)
 }
 
+// --- FX-hotspot classification (S-VIEW-BUG-1) ---------------------------------
+//
+// THE BUG: dropping a capture onto a track's FX button (in the TCP) never armed the
+// instrument drop, because the old predicate matched only "fx_" — which the SDK reserves for
+// the FX CHAIN / FLOATING-FX windows. A track-panel hit (where the FX button lives) reports a
+// string beginning with "tcp"/"mcp". These tests fix the classifier at the boundary the shell
+// consumes: each would FAIL under the old "fx_"-only rule for the tcp/mcp cases.
+
+// The regression case: the TCP FX-button surface reports "tcp*" and MUST classify as an FX
+// hotspot. Under the pre-fix "fx_"-only predicate these all returned false — the exact miss
+// that produced the "drops as audio to arrange" symptom.
+static void testTcpMcpPanelIsHotspot() {
+    CHECK(infoNamesFxHotspot("tcp"));       // bare track control panel
+    CHECK(infoNamesFxHotspot("tcp.fx"));    // the TCP FX-button WALTER element
+    CHECK(infoNamesFxHotspot("tcp.mute"));  // any tcp.* sub-element resolves to the track
+    CHECK(infoNamesFxHotspot("mcp"));       // mixer control panel
+    CHECK(infoNamesFxHotspot("mcp.fx"));    // the MCP FX area
+}
+
+// The FX chain / floating-FX windows (the surfaces the ORIGINAL predicate matched) still
+// classify as hotspots — the fix widens the rule, it does not regress the "fx_" surface.
+static void testFxWindowStillHotspot() {
+    CHECK(infoNamesFxHotspot("fx_chain"));  // FX chain window
+    CHECK(infoNamesFxHotspot("fx_0"));      // first FX, floating
+    CHECK(infoNamesFxHotspot("fx_12"));     // arbitrary floating-FX index
+}
+
+// Non-FX surfaces are NOT hotspots — a drop here is not an instrument drop (it would fall
+// through to the OS drag / no-op). Guards against over-broad matching (e.g. "spacer_0" must
+// not match despite living near the tracks; "arrange" is the audio-import surface).
+static void testNonFxSurfacesAreNotHotspot() {
+    CHECK(!infoNamesFxHotspot("arrange"));
+    CHECK(!infoNamesFxHotspot("spacer_0"));
+    CHECK(!infoNamesFxHotspot(""));          // pointer over nothing REAPER classifies
+    CHECK(!infoNamesFxHotspot("trans"));     // transport
+    CHECK(!infoNamesFxHotspot("envcp"));     // envelope control panel — a track thing, not FX
+}
+
 int main() {
     testBlobRoundTripsThroughInstrumentReader();
     testGuidLikeIdRoundTrips();
@@ -114,6 +152,10 @@ int main() {
     testBase64RoundTripAllBytes();
     testBase64DecodeRejectsMalformed();
 
+    testTcpMcpPanelIsHotspot();
+    testFxWindowStillHotspot();
+    testNonFxSurfacesAreNotHotspot();
+
     if (g_fail == 0) std::printf("All tests passed.\n");
     return g_fail ? 1 : 0;
 }