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.

workshop-baker/src/prebake/stage/hook.rs

@@ -57,7 +57,7 @@ pub async fn hook(
     ctx: &ExecutionContext,
     event_tx: EventSender,
 ) -> Result<(), ExecutionError> {
-    dbg!(ctx);
+    // dbg!(ctx);
     let hook = match hook {
         Some(h) => h,
         None => {
@@ -79,7 +79,7 @@ pub async fn hook(
         let result = engine
             .execute_command_with_delta(&hook_cmd.command, ctx, &event_tx, &deltactx)
             .await;
-        dbg!(&result);
+        // dbg!(&result);
         if let Err(e) = result {
             log::error!("Hook {} failed: {:?}", index, e);
             return Err(e);