An edge to a vanished port needs a handle, so the port stays as a ghost (#60)

Four ways erase a derived port and take no edge with them: deleting a state,
deleting the `State machine` literal, deleting `Behavior start`/`end`, deleting
the asset. The common cause is `RemoveNodeCommand` clearing only its own graph,
so the fix sits in `drawnDef` (graph/nested.ts) and touches no delete site.

A right-click on an edge goes through `view.portAt` only — there is no curve
hit-test. Without a port row such an edge is not merely invisible, it is
unreachable, so the ghost row is the handle Disconnect hangs on.

`resolveDef` stays untouched: it is the truth for interpreter, compiler,
ConnectCommand and dropTarget, so nothing runs or connects through a ghost
without a second guard.

src/core/graph/NodeRegistry.ts

@@ -16,6 +16,8 @@ export interface PortDef {
   field?: PropertyDef;
   /** `false`: nothing to type in even for a number – unconnected, the node's own rule applies. */
   inline?: boolean;
+  /** Not a port of the definition: a key only an edge still names, drawn so that edge can be seen. */
+  ghost?: true;
 }
 
 /**