Adds the `user.rust` UPM to workshop-engine for managing Rust
toolchains,
components, cross-compilation targets, and cargo operations:
- `rustup default <toolchain>` / `rustup component add` / `rustup target
add`
- `cargo fetch` for dependency pre-fetch with optional `--locked`
- `cargo install` for global crate tools with registry support
Includes 275-line test suite (`tests/upm/test_rust.py`) covering the
full
lifecycle with optional network tests.
Catty Steve