fix(bazel): ensure FTXUI is publicly accessible and add external smoke test (#1157)

- Set `visibility = ["//visibility:public"]` on the top-level `:ftxui` alias
  so the library can be consumed from external Bazel workspaces.
- Add `bazel_integration/` minimal external workspace to validate
  external usage via Bzlmod.
- Introduce `smoke` target that depends on `@ftxui//:ftxui`.
- Add CI job to build the smoke target using:
    --enable_bzlmod
    --override_module=ftxui=..
  This prevents regressions in visibility or public API changes.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

bazel/test/smoke.cpp

@@ -0,0 +1,16 @@
+// Copyright 2025 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.
+#include <ftxui/dom/elements.hpp>
+#include <ftxui/screen/screen.hpp>
+#include <ftxui/component/component.hpp>
+#include <ftxui/component/screen_interactive.hpp>
+
+int main() {
+  using namespace ftxui;
+  auto screen = ScreenInteractive::TerminalOutput();
+  auto component = Button("Quit", screen.ExitLoopClosure());
+  screen.Loop(component);
+  return 0;
+}
+