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_platform.cpp

@@ -244,6 +244,7 @@ LRESULT CALLBACK ReaSamplerEditor::wndProc(HWND hwnd, UINT msg, WPARAM wParam,
                     }
                     self->drag_ = DragKind::kNone;
                     self->dragParamId_ = -1;
+                    self->dragInnerCellId_ = -1;  // inner-dial drag state (peer reset)
                     self->curvePointIndex_ = -1;  // curve-node drag state (peer reset)
                     // No cursor position is available here to re-resolve hover (unlike
                     // onMouseUp's release coordinates), so clear rather than leave it naming