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.cpp

@@ -39,6 +39,14 @@ std::string buildInstrumentDropChunk(const std::string& sampleId) {
     return encodeBase64(instrumentDropStateBytes(sampleId));
 }
 
+bool infoNamesFxHotspot(const std::string& info) {
+    // See the header contract. The FX-chain / floating-FX windows report "fx_*"; the track /
+    // mixer panel that carries the FX button reports "tcp*" / "mcp*". Prefix-match all three —
+    // the TCP/MCP surfaces are the fix for S-VIEW-BUG-1 (the "fx_"-only predicate missed them).
+    auto startsWith = [&info](const char* p) { return info.rfind(p, 0) == 0; };
+    return startsWith("fx_") || startsWith("tcp") || startsWith("mcp");
+}
+
 std::string encodeBase64(const std::vector<std::uint8_t>& bytes) {
     std::string out;
     out.reserve(((bytes.size() + 2) / 3) * 4);