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/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). */