Adopt Rust example

Rust workspace with three path crates (api, support, worker) and a manifest-free solo directory.

bazel run @rules_dx//:dx -- init //examples/adopt-rust/...
bazel run @rules_dx//:dx -- generate //examples/adopt-rust/...
bazel build //examples/adopt-rust/...
bazel test //examples/adopt-rust/...

All 5 tests pass (api_test, api_roundtrip_test, common_test, helper_test, solo_test).