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/.bazelrc

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+../../.bazelrc

bazel/test/BUILD.bazel

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+# 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.
+
+# Test using individual submodules
+cc_binary(
+    name = "smoke",
+    srcs = ["smoke.cpp"],
+    deps = [
+        "@ftxui//:component",
+        "@ftxui//:dom",
+        "@ftxui//:screen",
+    ],
+)
+
+# Test using the single ftxui target
+cc_binary(
+    name = "smoke_single_dependency",
+    srcs = ["smoke.cpp"],
+    deps = [
+        "@ftxui",
+    ],
+)

bazel/test/MODULE.bazel

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+# 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.
+module(
+    name = "ftxui_integration_test",
+    version = "0.0.1",
+)
+
+bazel_dep(name = "ftxui", version = "6.1.9")

bazel/test/README.md

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+# FTXUI Bazel Integration Test
+
+This directory contains integration tests to verify that FTXUI can be properly consumed as an external dependency using Bazel with Bzlmod.
+
+## Purpose
+
+These tests ensure that:
+- FTXUI's public API is correctly exposed to external projects
+- Both single-target (`@ftxui//:ftxui`) and submodule-based dependencies work correctly
+- Headers are properly re-exported and accessible from downstream projects
+
+## Build Instructions
+
+To build all targets:
+
+```bash
+bazel build //... --enable_bzlmod --override_module=ftxui=../..
+```
+
+To build individual targets:
+
+```bash
+# Test using individual submodules
+bazel build //:smoke --enable_bzlmod --override_module=ftxui=../..
+
+# Test using the single ftxui target
+bazel build //:smoke_single_dependency --enable_bzlmod --override_module=ftxui=../..
+```
+
+## Run the Examples
+
+```bash
+# Run the submodules version
+./bazel-bin/smoke
+
+# Run the single-target version
+./bazel-bin/smoke_single_dependency
+```

bazel/test/WORKSPACE.bazel

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+# 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.
+workspace(name = "ftxui_smoke_test")

bazel/test/smoke.cpp

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+// 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;
+}
+