feat(effect_stack,timeline,node_graph): library drag-drop, params view caching, clip click select

- effect_stack: the params view is cached per effect (recreation per
  render killed in-progress slider drags and clicks); new
  LibraryEffectDrag payload + AddTypeRequested event let an effect
  dragged from the app's effect library drop onto the stack at the
  indicator position
- timeline: left-press selects clips (plain press selects an
  unselected clip, keeps multi-selection on a selected one for group
  drags, Ctrl/Cmd toggles membership)
- node_graph: graph_position_at() converts a window-space point (drop
  position) to graph space

crates/gpui/examples/learn/effect_stack.rs

@@ -197,6 +197,7 @@ impl StackDemoRoot {
 						EffectStackEvent::ReorderRequested { .. }
 						| EffectStackEvent::RemoveRequested(_)
 						| EffectStackEvent::AddRequested { .. }
+						| EffectStackEvent::AddTypeRequested { .. }
 						| EffectStackEvent::ContextMenuRequested { .. }
 						| EffectStackEvent::CardSelected { .. }
 						| EffectStackEvent::ParameterChanged { .. } => {