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 abstraction well. A few notes:

**`set_baudrate` error handling is odd due to the API design:**

```rust
if sdev
    .set_baudrate(
        dev.fwnode()
            .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
            .unwrap_or(115200),
    )
    .is_err()
{
    return Err(EINVAL);
}
```

This discards the actual baudrate returned in the error. If `set_baudrate` returned `u32` instead of `Result<(), u32>`, this would be simpler and more informative.

**Echo server with `Timeout::Max` would hang:** Due to the `Timeout::Max` bug (returning 0), `write_all` in receive would return immediately or with ETIMEDOUT rather than actually waiting. This makes the sample non-functional in practice.

**The sample stores an `ARef<serdev::Device>` but never uses it** (except in Drop for a debug message). This is fine for a sample but worth noting.

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Generated by Claude Code Patch Reviewer