Let a graph node collapse to its title bar

A chevron in front of the title toggles it without touching the selection,
and right-click offers Collapse node / Expand node for the whole selection
as one undo step. A collapsed node draws its title bar alone; every link of
one side meets at a single point on it, and hidden links there share one
mark per side, since per-port marks would stack on the same pixels. It has
no ports to grab, so a link dropped on it connects nothing.

Like hiding a link this is drawing only: collapsed is saved on the node and
an expanded node leaves no trace in the project file.

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

src/editor/debugHook.ts

@@ -191,6 +191,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
       portAt: (nodeId: string, port: string, isOutput: boolean) =>
         graphEditor.canvas.portGlobal(nodeId, port, isOutput),
       nodeAt: (nodeId: string) => graphEditor.canvas.nodeGlobal(nodeId),
+      chevronAt: (nodeId: string) => graphEditor.canvas.chevronGlobal(nodeId),
       nodeLabel: (nodeId: string) => graphEditor.canvas.nodeLabel(nodeId),
       nodeProblem: (nodeId: string) => graphEditor.canvas.nodeProblem(nodeId),
       nodeSettings: (nodeId: string) => graphEditor.canvas.nodeSettings(nodeId),