kaidokert / fixed-bigint-rs / 29712440137

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Add ct_select + Ct arms for Saturating*/AbsDiff on HeaplessBigInt (#169)

* Add ct_select and Ct arms for Saturating* / AbsDiff on HeaplessBigInt

Introduces `ct_select` (cmp.rs) — a thin wrapper over the existing
`subtle::ConditionallySelectable` whole-value masked select, the runtime
analog of FixedUInt's `const_ct_select` (heapless needs no const-fn variant
since its impls aren't const-evaluated). Result width is max(operand len),
choice-independent.

Uses it to add the branchless Ct arms for the immediate dependents that were
Nct-only for lack of a select: SaturatingAdd/Sub/Mul (pick the saturation
sentinel vs the wrapped result on the overflow/borrow flag) and AbsDiff
(pick a-b vs -(a-b) on the borrow). All additive — the Nct impls are
untouched, so no bound cascade into the test harness; the Ct impls carry the
extra `T: subtle::ConditionallySelectable` bound.

NextPowerOfTwo (the remaining const_ct_select dependent on FixedUInt) is a
more involved bit-scan/zero-handling case, left for a follow-up.

* Tidy Ct abs_diff trait calls to match arith.rs style

Drop the verbose <Self as const_num_traits::OverflowingSub>:: / <Self as
WrappingSub>:: qualified paths in favor of imported by-value trait calls
(OverflowingSub::overflowing_sub / WrappingSub::wrapping_sub), matching how
the Ct saturating impls are written in arith.rs. Method-call syntax
(self.overflowing_sub(&other)) can't be used here — the by-value trait
method shadows the by-ref inherent at the value receiver, so &other
mismatches the expected Self. The WrappingSub/Zero imports stay: Self::zero()
needs Zero and the negation needs WrappingSub. Addresses review on #169.

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