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>

README.md

@@ -425,9 +425,13 @@ cc_binary(
     name = "your_target",
     srcs = ["your_source.cc"],
     deps = [
+        # Choose submodules
         "@ftxui//:component",
         "@ftxui//:dom",
         "@ftxui//:screen",
+        
+        # Or use the single ftxui target (includes all modules)
+        # "@ftxui//:ftxui",
     ],
 )
 ```