loop: crossfade the Gate sustain seam, and unshadow the loop handles that made loop points look gone

src/core/instrument/map/sample_map.cpp

@@ -270,6 +270,7 @@ ResolvedCapture resolveCapture(const SelectedSample& ref, const InstrumentParams
     // The override wins over the intrinsic; absent -> intrinsic (loop) / frame 0 (start).
     // The bank is never mutated.
     rs.loop = params.loopOverride ? *params.loopOverride : ref.loop;
+    rs.loopCrossfadeFrames = params.loopCrossfadeFrames;
     rs.startFrame = params.startPoint ? *params.startPoint : 0;
     rs.play = params.play;  // SECONDS; buildSampleData resolves to frames
     return rs;
@@ -306,6 +307,7 @@ SampleData buildSampleData(const ResolvedCapture& resolved, DecodedPcm decoded)
     data.sampleRate = decoded.sampleRate;
     data.rootNote = resolved.rootNote;
     data.loop = resolved.loop;
+    data.loopCrossfadeFrames = resolved.loopCrossfadeFrames;
     data.startFrame = resolved.startFrame;
     data.keyTrack = resolved.keyTrack;
     data.velocityCurve = resolved.velocityCurve;