opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

apps/desktop/src/components/assistant-ui/message-render-boundary.tsx

@@ -1,48 +0,0 @@
-import { Component, type ReactNode } from 'react'
-
-// `@assistant-ui/store`'s index-keyed child-scope lookup (`tapClientLookup`)
-// throws — rather than returning undefined — when a subscriber reads an index
-// that the message/parts list no longer has. This races during high-frequency
-// store replacement (session switch mid-stream, gateway reconnect replay): a
-// subscriber from the previous, longer list is still in React's notification
-// queue and reads one slot past the new, shorter array before it can unmount.
-// The throw is transient and self-heals on the next consistent snapshot, but
-// without a local boundary it unwinds to the root and blanks the whole app.
-// Upstream-tracked: assistant-ui/assistant-ui#4051, #3652.
-const isTransientLookupError = (error: unknown): boolean =>
-  error instanceof Error && /tapClient(Lookup|Resource).*out of bounds/.test(error.message)
-
-interface Props {
-  // Changes whenever the message list mutates; remounting clears the caught
-  // error so the next consistent render recovers silently.
-  resetKey: string
-  children: ReactNode
-}
-
-export class MessageRenderBoundary extends Component<Props, { error: Error | null }> {
-  state: { error: Error | null } = { error: null }
-
-  static getDerivedStateFromError(error: Error) {
-    return { error }
-  }
-
-  componentDidUpdate(prev: Props) {
-    if (this.state.error && prev.resetKey !== this.props.resetKey) {
-      this.setState({ error: null })
-    }
-  }
-
-  render() {
-    if (this.state.error) {
-      // Only swallow the transient store race; re-throw anything else so real
-      // bugs still reach the root error boundary.
-      if (!isTransientLookupError(this.state.error)) {
-        throw this.state.error
-      }
-
-      return null
-    }
-
-    return this.props.children
-  }
-}