RUSTSEC-2026-0220 Uint shift operations: incorrect overflow flags and truncated shift amounts

Uint::overflowing_shl/overflowing_shr returned false-negative overflow flags. overflowing_shl missed bits shifted above BITS but within the top limb (non-limb-aligned widths such as U160), and limbs wholly discarded by shifts >= 64; overflowing_shr missed wholly discarded low limbs. Shifted values were correct; only the flag was wrong.

The wrong flag propagates: checked_shl/checked_shr return Some instead of None, strict_* fail to panic, and saturating_* return a wrapped value instead of saturating. The incorrect checked_shl result causes to_base_be (and string formatting) to loop forever on no-alloc builds for non-limb-aligned widths — a denial of service if formatting is reachable from untrusted input.

Separately, wrapping_shl/wrapping_shr on 64/128/256-bit types truncated the shift amount modulo 2^32, so shifts >= 2^32 returned an incorrectly wrapped value instead of zero; on 32-bit targets the generic path also truncated 64-bit shift amounts.

Callers using checked or saturating shift semantics on untrusted shift amounts may compute incorrect results.