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

src/core/instrument/map/sample_map.h

@@ -227,6 +227,12 @@ struct InstrumentParams {
     std::optional<SampleLoop> loopOverride;  // instrument-owned sustain loop; absent -> intrinsic
     std::optional<std::int64_t> startPoint;  // instrument-owned initial read frame; absent -> 0
 
+    // Pre-seam crossfade at the loop reset, in SOURCE frames — a source-timeline quantity
+    // like the loop points, so it needs no rate to resolve and cannot be rescaled by a
+    // project/file rate mismatch. 0 is the hard seam a blob predating the field lifts to.
+    // Never a bank fact: the fade is a performance choice, the loop points are the file's.
+    std::int64_t loopCrossfadeFrames = 0;
+
     // Key-tracking scalar: how far playback pitch tracks the keyboard around the root. 1.0
     // (100%, standard 12-tone-ET) is the default — a blob predating this field lifts to
     // exactly 1.0, so already-saved instances are bit-identical. 0.0 = no tracking (every
@@ -259,6 +265,7 @@ struct ResolvedCapture {
     double keyTrack = 1.0;
     VelocityCurve velocityCurve = VelocityCurve::flat();
     SampleLoop loop;            // effective: loopOverride, else bank intrinsic
+    std::int64_t loopCrossfadeFrames = 0;  // instrument-owned; no bank intrinsic to beat
     std::int64_t startFrame = 0;  // effective initial read frame: startPoint, else 0
     PlaySeconds play;           // stored SECONDS; resolved to frames at build
 };