fix: show the instances field in the inspector

src/components/Inspector.instances.test.tsx

@@ -0,0 +1,57 @@
+// @vitest-environment jsdom
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { act } from 'react';
+import { createRoot, type Root } from 'react-dom/client';
+import { Inspector } from './Inspector';
+import { makeNode } from '../sim/presets';
+import type { NodeKind } from '../sim/types';
+
+/**
+ * The inspector only renders fields that belong to one of its groups, so a
+ * field listed for a kind but left out of every group is silently dropped.
+ * `instances` was, which left scaling out unreachable from the panel even
+ * though the engine, the canvas badge and the autoscaler all honour it.
+ */
+
+let container: HTMLDivElement;
+let root: Root;
+
+beforeEach(() => {
+  (globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT =
+    true;
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  root = createRoot(container);
+});
+
+afterEach(() => {
+  act(() => root.unmount());
+  container.remove();
+  document.body.innerHTML = '';
+});
+
+function fieldLabels(kind: NodeKind): string[] {
+  act(() =>
+    root.render(
+      <Inspector
+        node={makeNode(kind, 0, 0)}
+        stats={null}
+        onChange={() => {}}
+        onDelete={() => {}}
+        onRename={() => {}}
+      />,
+    ),
+  );
+  return [...container.querySelectorAll('label')].map(
+    (l) => l.textContent?.trim() ?? '',
+  );
+}
+
+describe('the instances field in the inspector', () => {
+  it.each<NodeKind>(['service', 'db', 'cache', 'lb', 'worker'])(
+    'is editable on a %s',
+    (kind) => {
+      expect(fieldLabels(kind)).toContain('Instances');
+    },
+  );
+});

src/components/Inspector.tsx

@@ -1223,7 +1223,13 @@ const FIELD_GROUPS: { title: string; fields: ReadonlySet<Field> }[] = [
   },
   {
     title: 'How much it can handle',
-    fields: new Set<Field>(['capacity', 'serviceMs', 'serviceCv', 'queueLimit']),
+    fields: new Set<Field>([
+      'instances',
+      'capacity',
+      'serviceMs',
+      'serviceCv',
+      'queueLimit',
+    ]),
   },
   {
     title: 'When things go wrong',