feat(cmux-next): add focused sidebar collapse motion
What changed
Extract the focused sidebar collapse motion from the former mixed animation branch. The sidebar container now owns the collapse transition and tunables keep the prototype fade fast enough for workspace switching.
Scope
This PR contains only sidebar collapse/fade motion. Pane, tab, workspace-title, scrollbar, streaming-follow, and gallery evidence work are separate.
Validation
- Focused source diff reviewed against
feat-cmux-next - No native build run on the MacBook Air; fleet/CI owns native compilation
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Summary by cubic
Uses the shared sidebar resize handle so collapse and resize motion is the only behavior in this branch. SidebarResizeHandle stays invisible at rest, fades in a hairline on hover or while dragging, and keeps the resize cursor and hit area at all times. Also removes the sidebar.workspaceTitleMotion prototype tunable so title crossfade/lift ships separately.
Scope: this PR contains only sidebar collapse and resize motion; pane, tab, workspace-title, scrollbar, streaming-follow, and gallery work are separate.