feat(gpui): add dock, effect stack, node graph, and timeline widgets
Implements four workspace widget modules with accompanying learn examples:
- dock: dockable panel layout system (tabs, splits, drag-to-dock) with
serde-based persistence via PanelRegistry / DockLayoutState
- effect_stack: linear effect-stack inspector widget
- node_graph: node-graph editor (nodes, ports, wires, pan/zoom canvas)
- timeline: video-editing timeline (tracks, clips, ruler, playhead)
Timeline snapping prefers the earlier frame when two snap points are
equally close, with SnapKind priority breaking same-frame ties.
Mike-Solar
ad7c965c5a