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

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

Patch Review

The sample is clean and demonstrates the key API surface well.

**One issue with the sample probe:**
```rust
fn probe(
    sdev: &serdev::Device<Core>,
    _info: Option<&Self::IdInfo>,
) -> impl PinInit<Self, Error> {
    ...
    if sdev.set_baudrate(...).is_err() {
        return Err(EINVAL);
    }
```
The `set_baudrate` method is on `Device<Bound>`, but `sdev` is `&Device<Core>`. This works via the deref coercion chain, which is correct but could be surprising to readers. A brief comment in the abstraction's module-level docs explaining the device context hierarchy and auto-deref would help (though this is a broader Rust-for-Linux documentation issue, not specific to this patch).

**The receive callback echoes data back:**
```rust
fn receive(sdev: &serdev::Device<Bound>, _this: Pin<&Self>, data: &[u8]) -> usize {
    let _ = sdev.write_all(data, serdev::Timeout::Max);
    data.len()
}
```
This is fine for a sample. The `let _ =` discards the write result, which is acceptable for demonstration purposes. Note that this always returns `data.len()` (claiming all data was consumed) regardless of whether `write_all` succeeded, which is the right behavior for a simple echo.

**The `SampleDriver` stores an `ARef<serdev::Device>` but only uses it in `Drop`** for a debug message. This is fine as a sample demonstrating how to keep a reference to the device.

No blocking issues with this patch.

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Generated by Claude Code Patch Reviewer