Adds a widget test asserting each destination icon keeps its Y position
whether the rail is collapsed or expanded — a recurring regression. To
drive expansion without a hover gesture (which trips RenderFlex overflow
mid-transition), a test-only startSidebarExpanded flag threads
StudioApp -> StudioShell -> _Sidebar (controller starts at 1.0, hover
disabled); _SidebarItems get stable ValueKeys. analyze clean; new test
and the existing smoke test pass.
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>
