Let a link be hidden, and shown again by selecting either end

A link can be folded away from the canvas: instead of a curve running clear
across the graph, each end grows a short stub with a mark naming the node at
the far side. The curve comes back while either of its two nodes is selected,
so following a connection is a click rather than a search, and a link carrying
a live pulse is always drawn so the trace keeps its thread.

Marks are collected per port, not per link, because several hidden links on one
port used to stack stubs, dots and labels on identical pixels. A bundle reads
"Move +1" and its dot selects every node the port feeds, as one undo step.

The whole thing is one optional flag on the edge; the interpreter never sees it.

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

src/core/store/serialize.ts

@@ -240,6 +240,7 @@ function normalizeGraph(raw: unknown): Graph {
       from: { node: from.node, port: from.port },
       to: { node: to.node, port: to.port },
     };
+    if (e.folded === true) edge.folded = true;
     graph.edges.push(edge);
   }
   return graph;