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>

.gitignore

@@ -24,6 +24,7 @@ out/
 !BUILD.bazel
 !MODULE.bazel
 !.bazelrc
+!.bazelignore
 
 # .github directory:
 !.github/**/*.yaml
@@ -37,6 +38,10 @@ out/
 # bazel directory:
 !bazel/**/*.bzl
 !.bcr/*
+!bazel/test/*.bazel
+!bazel/test/*.bazelrc
+!bazel/test/*.cpp
+!bazel/test/*.md
 
 # doc directory:
 !doc/**/Doxyfile.in