One taper, one modifier law: extract param_taper, raise the stage ceiling to 10 s, and make the AHDSR schematic axis the taper itself

src/shell/instrument/reasampler_editor.h

@@ -519,6 +519,12 @@ private:
     // delta from this anchor, so a grab never jumps the value.
     double dragKnobStartValue_ = 0.0;
 
+    // The modifier state the in-flight drag is anchored to. Every transition of it — press OR
+    // release — RE-ANCHORS the drag: current value and current cursor become the new origin, so
+    // the value is continuous across the flip and only the rate changes. Without that, rescaling
+    // an accumulated absolute delta in place jumps by (1 - kFineDragScale) x the accumulation.
+    instrument::ui::DragModifiers dragMods_{};
+
     // Which velocity curve the popup is editing; kNone = closed. Never persisted. Every writer
     // of kNone must also cancel a live curve-node drag (closeCurvePopup does both) — an Esc
     // mid-drag that closed the popup without cancelling the drag used to leave editedCurve()'s