Claude review: rust: dma: introduce dma::CoherentBox for memory initialization

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

Patch Review

This is the key safety improvement. `CoherentBox<T>` wraps `Coherent<T>` and provides safe `Deref`/`DerefMut` since the DMA address hasn't been exposed to hardware yet.

**`From<CoherentBox<T>> for Coherent<T>`**: The conversion at:
```rust
impl<T: AsBytes + FromBytes + KnownSize + ?Sized> From<CoherentBox<T>> for Coherent<T> {
    fn from(value: CoherentBox<T>) -> Self {
        value.0
    }
}
```
This moves the inner `Coherent` out without `Drop` running on the `CoherentBox`, which is correct since `CoherentBox` is a newtype tuple struct and the compiler handles the move semantics.

**`init_at` bounds check**: The `init_at` method checks `i >= self.0.len()` before accessing `&raw mut self[i]`. This is correct. The `Init` closure is run on valid, aligned memory that's zeroed. The safety comment correctly notes that `AsBytes + FromBytes` means partial init on error is not a problem.

**No way to get DMA handle from `CoherentBox`**: This is intentional and the whole point — you can't give the address to hardware until converting to `Coherent`. Good design.

---
Generated by Claude Code Patch Reviewer