Claude review: rust: extract `bitfield!` macro from `register!`

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

Patch Review

**Review: Looks good, minor nits**

The extraction is mechanically clean -- the code in `rust/kernel/bitfield.rs` is essentially identical to what was previously in `rust/kernel/io/register.rs`, with no functional changes to the macro logic itself.

**MAINTAINERS ordering (nit):** The new `RUST [BITFIELD]` entry is placed before `RUST [ALLOC]`, but alphabetically `[ALLOC]` < `[BITFIELD]`. The entry should be placed after `RUST [ALLOC]`:

```
RUST [ALLOC]
...
RUST [BITFIELD]    <-- should come here
```

**MASK computation correctness:** The mask formula at line ~731 of the new file:
```rust
$vis const [<$field:upper _MASK>]: $storage =
    ((((1 << $hi) - 1) << 1) + 1) - ((1 << $lo) - 1);
```
This avoids the `1 << BITS` overflow by computing `((1 << (hi)) - 1) << 1) + 1` instead of `(1 << (hi + 1)) - 1`. Verified correct for edge cases including bit 0 and the MSB.

**Zeroable safety (looks correct):** The safety comment:
```rust
// SAFETY: `$storage` is `Zeroable` and `$name` is transparent.
unsafe impl ::pin_init::Zeroable for $name {}
```
This is sound because all unsigned integer types are `Zeroable` and the struct is `#[repr(transparent)]`, so the all-zeroes representation is valid.

**No `Default` impl:** Unlike the old nova-core local `bitfield!` macro, the new kernel-level macro does not generate a `Default` implementation -- only `Zeroable`. This is fine since `zeroed()` is the equivalent for numeric types, but users migrating from local bitfield macros that provided `Default` should be aware. The commit message could mention this explicitly.

**No overlap detection:** The macro does not prevent defining overlapping bit ranges for different fields (e.g., `7:4 field_a; 5:4 field_b;`). This is arguably a feature (sometimes you want different views of the same bits), but it may be worth adding a note in the module documentation that overlapping fields are allowed and mutating one may affect another.

**register! delegation:** The `register!` macro changes at line ~903 correctly delegate to `bitfield!`:
```rust
$crate::bitfield!(
    #[allow(non_camel_case_types)]
    $(#[$attr])* $vis struct $name($storage) { $($fields)* }
);
```
This properly forwards all attributes, visibility, and field definitions.

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Generated by Claude Code Patch Reviewer