opentui(bench): scripts/demo.tsx — view the fixture in a real attachable TUI

Dev demo (not a test): seeds the bench fixture into the store via the resume path
and renders <App> under a real CliRenderer (no gateway) so you can attach over
tmux, scroll, and eyeball the transcript + the rolling-cap truncation notice.
Run: DEMO_TOTAL=240 HERMES_TUI_MAX_MESSAGES=80 bun scripts/demo.tsx

ui-tui-opentui-v2/scripts/demo.tsx

@@ -0,0 +1,46 @@
+/**
+ * DEV DEMO — NOT a test, NOT production. Renders the bench fixture (lorem-ipsum +
+ * fat tool-turns from ./fixture.ts) in a REAL CliRenderer so you can attach over
+ * tmux, scroll, and eyeball the transcript + the rolling-cap truncation notice.
+ * No gateway is spawned (purely the fixture seeded into the store via the resume
+ * path), so typing won't reach a backend — it's for viewing/scrolling.
+ *
+ * Run:  bun scripts/demo.tsx
+ *   DEMO_TOTAL=200               fixture messages to seed (default 200)
+ *   HERMES_TUI_MAX_MESSAGES=80   cap → the "⤒ N earlier messages" notice fires
+ * Quit: Ctrl+C.
+ */
+import { createCliRenderer } from '@opentui/core'
+import { render } from '@opentui/solid'
+
+import { createSessionStore } from '../src/logic/store.ts'
+import { App } from '../src/view/App.tsx'
+import { ThemeProvider } from '../src/view/theme.tsx'
+import { materialize } from './fixture.ts'
+
+const TOTAL = Number.parseInt(process.env.DEMO_TOTAL ?? '', 10) || 200
+
+const store = createSessionStore()
+store.apply({ type: 'gateway.ready' })
+store.setSessionId('demo-fixture-20260609')
+// Seed via the resume path so the cap slices + the `dropped` counter is set
+// (drives the truncation notice) exactly as a real `session.resume` would.
+store.beginBuffer()
+store.commitSnapshot(materialize(TOTAL))
+
+const renderer = await createCliRenderer({
+  externalOutputMode: 'passthrough',
+  targetFps: 60,
+  exitOnCtrlC: true,
+  useKittyKeyboard: {},
+  useMouse: true
+})
+
+void render(
+  () => (
+    <ThemeProvider theme={() => store.state.theme}>
+      <App store={store} />
+    </ThemeProvider>
+  ),
+  renderer
+)