Claude review: gpu: nova-core: convert to new dma::Coherent API

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

Patch Review

Final cleanup converting all remaining `CoherentAllocation` usage to `Coherent`.

**`from_data` safety concern in `dma.rs`**:
```rust
unsafe { dma_obj.as_mut()[..data.len()].copy_from_slice(data) };
```
`dma_obj` is a `DmaObject` which `Deref`s to `Coherent<[u8]>`. The `as_mut()` call is `Coherent::as_mut(&self)` returning `&mut [u8]`. The safety justification ("we are the only users and we haven't made the device aware of the handle yet") is valid. However, the allocation may be larger than `data.len()` due to page alignment in `DmaObject::new()`, so the `[..data.len()]` slice is correct.

**`DerefMut` for `DmaObject` inconsistency**: The patch changes `Deref::Target` to `Coherent<[u8]>` explicitly but doesn't touch the `DerefMut` impl, which still references `CoherentAllocation<u8>`. While this resolves to the same type via the type alias, it's inconsistent. Either both should use the new name or the `DerefMut` should also be updated. Minor nit.

**`dma_read!`/`dma_write!` simplification in `gsp/fw.rs`**: The conversion from `dma_read!(qs, [0]?.field)` to `dma_read!(qs, .field)` is clean and removes the fallible index + unwrap pattern that was previously needed when treating a single-element `CoherentAllocation` as a slice. This is a substantial readability improvement.

**Added `FromBytes` for `PteArray`**: The new `unsafe impl<const NUM_ENTRIES: usize> FromBytes for PteArray<NUM_ENTRIES> {}` is correct since `PteArray` is `[u64; N]` which has no invalid bit patterns.

**`IntoSafeCast` removal in `falcon.rs`**: The import of `IntoSafeCast` is dropped and the `dma_handle_with_offset` call is replaced with:
```rust
dma_obj.dma_handle() + DmaAddress::from(load_offsets.src_start)
```
This is simpler and avoids the fallible `dma_handle_with_offset` call. The arithmetic correctness depends on `src_start` being within bounds, which should be guaranteed by the caller context.

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