perf(shell): prevent workspace LayoutBuilder and layout rebuilds during sidebar toggle (#984)

[ZhuchkaTriplesix]

Closes #984

Summary

Resolves the remaining scope of #984 (following Part 1's #1004 LazyOffstage):

• In _MainContentSplitState, during _widthSpring animation ticks (toggleSidebar and drag flings), Expanded previously computed a new width on each frame (maxWidth - currentW - currentHandleW), forcing full performLayout passes down the visible workspace and rebuilding active LayoutBuilder elements (WorkspaceEmptyHero, VirtualResultGrid, DataGridStagingToolbar, etc.) on every single tick (~3.4–4.6ms per frame).
• Decouples workspace laid-out constraints from intermediate animation frames using ClipRect and OverflowBox(alignment: Alignment.topLeft), holding constraints stable at constraints.maxWidth while animating. Flutter's RenderObject.layout early-exits in O(1) across intermediate frames, eliminating per-frame reflows and LayoutBuilder rebuilds completely.
• When settled at rest, the workspace lays out against the final settled width once.
• Interactive mouse dragging retains live constraint updates.
• Added comprehensive widget test in sidebar_toggle_test.dart verifying stable constraints across intermediate ticks for both collapse and expand animations.