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

src/core/instrument/engine/play_params.h

@@ -170,6 +170,12 @@ struct SampleData {
     int rootNote = 60;
     SampleLoop loop;
 
+    // Pre-seam crossfade at the loop reset, in SOURCE frames — a source-timeline quantity
+    // like the loop points it belongs to, so no rate resolves it. 0 (the default) is the
+    // hard seam every instance predating the field plays. engine/loop/loop_span.h owns what
+    // the fade actually does and how it clamps.
+    std::int64_t loopCrossfadeFrames = 0;
+
     // Frame offset a voice starts playback at; frame 0 default is the pre-existing behavior.
     // Clamped into [0, frames) at note-on — a start >= sample length is a no-op (starts at 0).
     std::int64_t startFrame = 0;