Claude review: rust: io: add copying methods

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

Patch Review

Adds the `IoCopyable` trait and copy methods on `View`: `copy_read`, `copy_write`, `copy_from_slice`, `copy_to_slice`, and cross-I/O-region copying (`copy_from_io_slice`).

The `IoCopyable` trait differentiates between mapped (CPU-accessible) and unmapped (MMIO) backends:
- `Coherent`: `IS_MAPPED = true`, uses `memcpy`
- `Mmio`: `IS_MAPPED = false` (default), uses `memcpy_fromio`/`memcpy_toio`

The cross-I/O copy with a stack buffer fallback is a pragmatic approach:
```rust
fn copy_from_io_slice_via_buffer<T: ...>(&self, data: View<'_, T, [u8]>) {
    let mut buf = MaybeUninit::<[u8; 256]>::uninit();
    // ... copy in 256-byte chunks via the stack buffer
}
```

Two minor observations:

1. Typo in doc comments: "gurantee" appears twice (in `copy_read` and `copy_write`), should be "guarantee".

2. In `copy_from_io_slice_via_buffer`, the safety comment for the second `unsafe` block says "valid for write for `copy_len` bytes" but this is `copy_to_io` which writes to `dst_ptr` -- the comment seems correct functionally but says the buffer is "valid for write" when it should say "valid for read" since we're reading from the buffer at that point:
```rust
// - `buf.as_mut_ptr()` is valid for write for `copy_len` bytes as `copy_len <= 256`.
unsafe { self.io.copy_to_io(dst_ptr, buf.as_ptr().cast(), copy_len) };
```
The `.as_ptr()` call is correct (reading from buffer), but the comment says "valid for write" which is technically about `dst_ptr`, not `buf`. The comment seems to be a copy-paste from the block above. Minor, but worth fixing for accuracy.

Overall, this is a well-designed and well-executed series that significantly improves the Rust I/O abstractions in the kernel. The type safety improvements and the elimination of the `gsp_mem` workaround module demonstrate clear practical value.

---
Generated by Claude Code Patch Reviewer