fix(instrument_drop_win): UTF-8 temp path, PID collision guard, embed-strip excluded from drop hotspot

tests/test_instrument_drop.cpp

@@ -218,6 +218,20 @@ static void testNonFxSurfacesAreNotHotspot() {
     CHECK(!infoNamesFxHotspot("envcp"));     // envelope control panel — a track thing, not FX
 }
 
+// The embed-strip sub-element is NOT a hotspot. "tcp.fxembed" / "mcp.fxembed" is the surface
+// where a ReaSampler 9000 instance draws inline in the TCP/MCP via IReaperUIEmbedInterface.
+// Dropping a card there must NOT add a SECOND instance on top of the existing embed — the
+// drop should be ignored (no instrument drop), even though the token starts with "tcp.fx".
+// This documents the explicit exclusion in infoNamesFxHotspot and would catch a regression if
+// the exclude guard were accidentally removed.
+static void testEmbedStripIsNotHotspot() {
+    CHECK(!infoNamesFxHotspot("tcp.fxembed"));   // TCP embed strip — existing instance's surface
+    CHECK(!infoNamesFxHotspot("mcp.fxembed"));   // MCP embed strip — existing instance's surface
+    // With hypothetically appended info (SDK "may append") — still excluded.
+    CHECK(!infoNamesFxHotspot("tcp.fxembed.1"));
+    CHECK(!infoNamesFxHotspot("mcp.fxembed extra"));
+}
+
 int main() {
     testClassIdHexPinnedPerChannel();
     testPresetRoundTripsThroughInstrumentReader();
@@ -230,6 +244,7 @@ int main() {
     testTcpMcpFxFamilyIsHotspot();
     testFxWindowStillHotspot();
     testNonFxSurfacesAreNotHotspot();
+    testEmbedStripIsNotHotspot();
 
     if (g_fail == 0) std::printf("All tests passed.\n");
     return g_fail ? 1 : 0;