Migrate XLS to Bazel 9

Now that #2941 is done, we should start planning on migrating to Bazel 9, the active LTS version at the time of this writing. Bazel 8 has entered maintenance mode.

Major differences are documented here: https://blog.bazel.build/2026/01/20/bazel-9.html

Since we are fully on bzmod, probably the main blocker is the Starlarkification of language rules.

Start by reviewing the completed work in #2941 and the linked Bazel 9 migration notes. Inspect the repository's bzmod setup and identify the language rules blocked by Starlarkification. Done means XLS builds and its existing checks pass under Bazel 9, with the migration blockers resolved.

Written by the indexing model from the issue text.