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/editor/graph/NodeView.ts

@@ -82,6 +82,11 @@ export function portColor(type: PortType): number {
   }
 }
 
+function rowStyle(port: PortDef): TextStyleToken {
+  if (port.ghost) return 'warning';
+  return port.type === 'exec' ? 'body' : 'muted';
+}
+
 export interface PortHit {
   port: PortDef;
   isOutput: boolean;
@@ -336,7 +341,7 @@ export class NodeView extends Widget {
       if (input) {
         const text = add(
           this.inputText(input),
-          input.type === 'exec' ? 'body' : 'muted',
+          rowStyle(input),
           PAD + PORT_RADIUS + 4,
           rowTop,
           'left',
@@ -347,13 +352,13 @@ export class NodeView extends Widget {
       if (output) {
         const text = add(
           output.label,
-          output.type === 'exec' ? 'body' : 'muted',
+          rowStyle(output),
           this.w - PAD - PORT_RADIUS - 4,
           rowTop,
           'right',
           NODE_WIDTH / 2 - 10,
         );
-        if (output.type !== 'exec')
+        if (output.type !== 'exec' && !output.ghost)
           this.valueSlots.set(output.key, { left: inputEnd + 6, right: text.x - 6, top: rowTop + 1 });
       }
     }
@@ -394,7 +399,10 @@ export class NodeView extends Widget {
         this.ports.circle(x, y, r + 2).stroke({ width: 2, color: c.node });
         this.ports.circle(x, y, r + 4).stroke({ width: 2, color: c.portTarget });
       }
-      if (port.type === 'exec') {
+      if (port.ghost) {
+        // Hollow: a pin is drawn where a port used to be, and nothing stands behind it any more.
+        this.ports.circle(x, y, r).stroke({ width: 2, color: c.danger });
+      } else if (port.type === 'exec') {
         // A pentagon pointing right: the Blueprint-style exec pin.
         this.ports
           .moveTo(x - r, y - r)