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/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>();