instrument: one staged-envelope system — per-segment curves, the sustain-less AHD, and a shared overlay for all three envelopes

Trigger's fade pair folds into the AHD (and goes live); the release anchors right;
Preserve rings its synthetic tail out instead of cutting it. Payload v10.

src/shell/instrument/editor_input.cpp

@@ -78,6 +78,7 @@ void ReaSamplerEditor::onMouseUp(int x, int y) {
     const Rect curveRect = dragCurveRect_;
     drag_ = DragKind::kNone;
     dragParamId_ = -1;
+    dragInnerCellId_ = -1;
     curvePointIndex_ = -1;
     // hover_ is deliberately not re-resolved during a drag (see resolveHover's caller), so it
     // still names wherever the drag started. Re-resolve now against the release position, for