feat(upm): implement Rust user package manager

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.

.gitignore

@@ -18,3 +18,5 @@ tests/**/__pycache__/
 tests/.pytest_cache/
 tests/llm/.cache/
 *.pyc
+.staging
+.pytest_cache