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

src/shell/instrument/reasampler_editor.h

@@ -93,8 +93,11 @@ private:
     using ParamControl = instrument::ui::DeckParam;
 
     // The waveform markers on the waveform band: start-point + the sustain loop's two ends,
-    // in draw + hit order.
-    enum class WaveMarker { kStart = 0, kLoopStart = 1, kLoopEnd = 2, kCount = 3 };
+    // in draw + hit order, then the crossfade handle. The crossfade is NOT part of the
+    // full-height column hit-test — it answers only in its top-strip handle (waveform_view's
+    // markerHandleRect), because at a zero fade it sits exactly on the loop start.
+    enum class WaveMarker { kStart = 0, kLoopStart = 1, kLoopEnd = 2, kLoopXfade = 3,
+                            kCount = 4 };
 
     // The interactive element under the pointer, resolved live in WM_MOUSEMOVE. `index`
     // disambiguates within a kind (tab ordinal, visible-card index, control-row id); -1 when
@@ -291,12 +294,15 @@ private:
     std::string samplePathFor(const std::string& sampleId) const;
 
     // The effective loop + start markers for the loaded capture: the parameter set's
-    // override when one is set, else the bank's loop intrinsic / frame 0. Absent loop ->
-    // loopStart==loopEnd==0. `frames` defaults loopEnd when the bank left the loop empty.
+    // override when one is set, else the bank's loop intrinsic / frame 0. With no loop set,
+    // the loop handles park on loop_span's defaultLoopBounds so both stay grabbable — the
+    // frame-0 default they replace put loopStart under the start marker, where nothing could
+    // reach it.
     struct SetupMarkers {
         std::int64_t start = 0;
         std::int64_t loopStart = 0;
         std::int64_t loopEnd = 0;
+        std::int64_t crossfade = 0;  // pre-seam fade, SOURCE frames; handle at loopStart - this
         bool hasLoop = false;  // whether a sustain loop is set (drives the "no loop" affordance)
     };
     SetupMarkers pickedMarkers(std::int64_t frames) const;
@@ -305,6 +311,10 @@ private:
     // callers decide live-drag vs final commit.
     void applyMarkers(const SetupMarkers& m);
 
+    // Arms a waveform-marker drag: the grabbed marker plus the snapshot the pixel-delta
+    // resolver and the inter-marker clamps measure from.
+    void beginMarkerDrag(WaveMarker which, const SetupMarkers& m, std::int64_t frames, int x);
+
     // Deck knobs edit the parameter set's PlaySeconds (play mode + AHDSR; pitch engine + AD
     // pitch envelope) — wall-clock seconds, rate-free; the build resolves to frames.