Fix drag-out losing audio and FX-container drops losing the capture; re-home ingest under shell/actions

src/core/wire/instrument_drop.cpp

@@ -76,6 +76,17 @@ std::vector<std::uint8_t> buildInstrumentDropPreset(const std::string& sampleId)
     return buildVstPresetBytes(vstClassIdHex(), instrumentDropStateBytes(sampleId));
 }
 
+DropOutcome decideDropOutcome(const DropAttempt& attempt) {
+    DropOutcome out;
+    if (attempt.addedFxIndex < 0) return out;  // add failed — nothing exists to roll back
+    if (!attempt.presetApplied) {
+        out.rollbackFxIndex = attempt.addedFxIndex;
+        return out;
+    }
+    out.loaded = true;
+    return out;
+}
+
 bool infoNamesFxHotspot(const std::string& info) {
     // See the header contract for the prefix rule and the embed-strip exclusion.
     auto startsWith = [&info](const char* p) { return info.rfind(p, 0) == 0; };

src/core/wire/instrument_drop.h

@@ -68,4 +68,25 @@ bool infoNamesFxHotspot(const std::string& info);
 // assert the capture is selected. Not called by the shell.
 std::vector<std::uint8_t> instrumentDropStateBytes(const std::string& sampleId);
 
+// --- All-or-nothing rollback --------------------------------------------------
+
+// What the shell observed while executing one drop, reduced to the two REAPER
+// results the contract turns on.
+struct DropAttempt {
+    int  addedFxIndex = -1;    // TrackFX_AddByName's return; < 0 = nothing was created
+    bool presetApplied = false;  // TrackFX_SetPreset's return
+};
+
+// The verdict. `rollbackFxIndex >= 0` obliges the caller to TrackFX_Delete it before
+// returning — an instance whose capture never landed must not survive the drop.
+struct DropOutcome {
+    bool loaded = false;
+    int  rollbackFxIndex = -1;
+};
+
+// Pure so the contract is provable without a DAW: the rollback obligation is decided
+// here, not inline in the shell, and holds identically for every drop surface (FX
+// button, FX chain/container, Media-Explorer import).
+DropOutcome decideDropOutcome(const DropAttempt& attempt);
+
 }  // namespace reasampler::wire