Revise the mock — toggles no longer float on the pane edges; they live in
the bottom status bar, pinned to the center pane's left/right edge so each
slides to the far end when its pane collapses.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Add the wireframe (states A/B) for edge-anchored collapse toggles on the
center pane. Records that collapse logic already exists (sidebar.collapse /
context.collapse commands) so the ticket is scoped to the mouse affordance.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>