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/README.md

@@ -0,0 +1,38 @@
+# 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
+```