Claude review: rust: io: use pointer types instead of address

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

Patch Review

The biggest refactor: replaces `addr() -> usize` / `maxsize() -> usize` with `as_ptr() -> *mut Self::Type` on the `Io` trait, and makes `IoCapable` take `*mut T` instead of `usize`. This eliminates `IoKnownSize` (kept as a blanket impl for compatibility).

The PCI config space handling is notable. Since PCI config space uses offsets not real pointers, null pointers with size metadata are used:
```rust
fn null_ptr_from_size(size: ConfigSpaceSize) -> *mut Self::Region {
    core::ptr::slice_from_raw_parts_mut(core::ptr::null_mut::<u8>(), size.into_raw())
        as *mut Self::Region
}
```

This is documented as intentional since `IoCapable` impls for PCI config space only use `address.addr()` (the offset), never dereferencing the pointer. The `io_addr` method now uses `wrapping_byte_add` which is appropriate for potentially-null pointers.

One minor observation: the `io_addr_assert` method now uses `build_assert!` with `Self::Type::MIN_SIZE` -- this means it checks against the compile-time minimum, while `io_addr` checks against the runtime `KnownSize::size(ptr)`. This is consistent with the intent (compile-time vs runtime bounds checking).

The `IoKnownSize` deprecation is handled gracefully with a blanket impl and `#[doc(hidden)]`.

---
Generated by Claude Code Patch Reviewer