Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

A clean sample that demonstrates probe, receive (echo/loopback), and cleanup. A few observations:

**Error in receive is swallowed**:

```rust
+    fn receive(sdev: &serdev::Device<Bound>, _this: Pin<&Self>, data: &[u8]) -> usize {
+        let _ = sdev.write_all(data, serdev::Timeout::Max);
+        data.len()
+    }
```

The `write_all` error is discarded with `let _ = ...`. For a sample this is acceptable, but a comment like `// Ignore write errors for simplicity` would be helpful.

**Strict baudrate matching**:

```rust
+        if sdev
+            .set_baudrate(...)
+            .is_err()
+        {
+            return Err(EINVAL);
+        }
```

This fails probe if the exact baudrate isn't matched. As noted above, many serial controllers approximate the baudrate. For a sample this is fine, but it could be confusing if someone tries to run this on real hardware.

**License string**: `"GPL v2"` is used, which is the correct form for `MODULE_LICENSE`.

**Overall the sample is clean and well-structured**, demonstrating the key abstractions (OF/ACPI ID tables, probe, receive, device reference management via `ARef`, Drop-based cleanup).

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