Rust standard boundary

This subroot applies wn-dev-std 2026.8.12 rust-app policy without replacing the repository Python-package boundary.

Generated files are checked in and excluded from structural source limits, but remain subject to compile, test, schema-conformance, and freshness gates. No vendored or unsafe project code is currently allowed.

uvx --from wn-dev-std==2026.8.12 dev-std audit rust --scope repo --scope language