`root`: The division can be turned into bit-shifts when the degree

*On 2026-08-31 @decofe wrote in [`a9450b4`](https://github.com/alloy-rs/ruint/commit/a9450b499c03f259238c2955b9512de873a77e12) “chore: apply latest nightly rustfmt (#627)”:*

The division can be turned into bit-shifts when the degree
is a power of two.

```rust
let mut decreasing = false;
loop {
// OPT: This could benefit from single-limb multiplication
// and division.
//
// OPT: The division can be turned into bit-shifts when the degree
// is a power of two.
let division = result
.checked_pow(deg_m1)
.map_or(Self::ZERO, |power| self / power);
let iter = (division + deg_m1 * result) / Self::from(degree);
match (decreasing, iter.cmp(&result)) {

```
*From [`src/root.rs:65`](https://github.com/alloy-rs/ruint/blob/a9450b499c03f259238c2955b9512de873a77e12/src/root.rs#L65)*

Start in src/root.rs around line 65 and trace the root iteration involving the degree and division. Check how power-of-two degrees can use bit-shifts, then verify that root results remain unchanged for those degrees and that the existing behavior is preserved for other degrees.

Written by the indexing model from the issue text.