Fix findUsages/undo/node-lookup bugs, split Viewport and Editor, share command and inspector helpers

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/graph/Graph.ts

@@ -44,10 +44,6 @@ export function findNode(graph: Graph, nodeId: string): GraphNode | undefined {
   return graph.nodes.find((n) => n.id === nodeId);
 }
 
-export function findEdge(graph: Graph, edgeId: string): GraphEdge | undefined {
-  return graph.edges.find((e) => e.id === edgeId);
-}
-
 /** Edges arriving at one input port (data inputs allow at most one, exec inputs any number). */
 export function edgesInto(graph: Graph, nodeId: string, port: string): GraphEdge[] {
   return graph.edges.filter((e) => e.to.node === nodeId && e.to.port === port);