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/core/commands/graphCommands.ts

@@ -242,6 +242,36 @@ export class FoldEdgeCommand implements Command {
   }
 }
 
+/** One undo step for the whole selection; every node is found before any of them changes. */
+export class CollapseNodesCommand implements Command {
+  readonly label: string;
+  private previous: boolean[] = [];
+
+  constructor(
+    private readonly assetId: string,
+    private readonly nodeIds: readonly string[],
+    private readonly collapsed: boolean,
+  ) {
+    const noun = nodeIds.length === 1 ? 'node' : 'nodes';
+    this.label = collapsed ? `Collapse ${noun}` : `Expand ${noun}`;
+  }
+
+  execute(project: Project): void {
+    const nodes = this.nodeIds.map((id) => requireNode(project, this.assetId, id));
+    this.previous = nodes.map((n) => n.collapsed === true);
+    for (const node of nodes) setCollapsed(node, this.collapsed);
+  }
+
+  undo(project: Project): void {
+    this.nodeIds.forEach((id, i) => setCollapsed(requireNode(project, this.assetId, id), this.previous[i]!));
+  }
+}
+
+function setCollapsed(node: GraphNode, collapsed: boolean): void {
+  if (collapsed) node.collapsed = true;
+  else delete node.collapsed;
+}
+
 export class DisconnectCommand implements Command {
   readonly label = 'Disconnect';
   private removed: Removed<GraphEdge>[] = [];

src/core/graph/Graph.ts

@@ -24,6 +24,8 @@ export interface GraphNode {
    * behavior its signature; an Expand/Pack node is what gives a struct its fields.
    */
   ports?: DeclaredPort[];
+  /** Drawn as its title bar alone, every link meeting at one point per side. Purely visual. */
+  collapsed?: boolean;
 }
 
 /** A port a start/end node declares. `kind` maps to the PortType of the same name (`text` → string). */

src/core/store/serialize.ts

@@ -244,6 +244,7 @@ function normalizeGraph(raw: unknown): Graph {
     const node: GraphNode = { id, type, x: toNumber(n.x), y: toNumber(n.y), params };
     const ports = normalizePorts(n.ports);
     if (ports.length > 0) node.ports = ports;
+    if (n.collapsed === true) node.collapsed = true;
     graph.nodes.push(node);
   }
   const edgeIds = new Set<string>();