kaidokert / fixed-bigint-rs / 29693523494

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CheckedDiv/CheckedRem + saturating family for HeaplessBigInt (#152)

* Add CheckedDiv/CheckedRem and the saturating family to HeaplessBigInt

Two of the three remaining num_traits::PrimInt blockers (the third is Num /
from_str_radix).

CheckedDiv/CheckedRem (div_rem.rs, Nct — division is Nct-only): return None
on divide-by-zero instead of the div_rem_impl assert, so div-by-zero is
reachable without the panicking operator. Inherent + const_num_traits +
num_traits trait forms.

Saturating (arith.rs): inherent saturating_add/sub (P-generic) and
saturating_mul, const_num_traits Saturating{Add,Sub,Mul} (Nct, matching the
Checked* gating), and num_traits::Saturating (Nct). Correctness point: the
saturation target is the all-ones value at the OPERAND width max(a.len,b.len)
via max_at_len, NOT Bounded::max_value() — on this carrier that's the CAP-wide
max, which would over-saturate a sub-capacity value. FixedUInt can use
Bounded::max_value() because its width IS N; heapless can't.

Tests: generic bodies for both carriers in carrier_generic.rs (const traits)
and carrier_num_traits.rs (num_traits bridges), plus heapless-internal tests
that the fixed-width harness can't reach — saturation clamps to the operand
width (len 1 in a CAP-8 carrier), not CAP, and checked div/rem by zero is None.

* Gate HeaplessBigInt saturating_* to Nct (Ct branch leaks saturation)

The inherent saturating_add/sub/mul were on the P-generic impl block, so the
Ct personality got them too — and they branch on the (secret-derived) overflow
flag, leaking whether saturation happened. FixedUInt keeps its Ct saturating
branchless via const_ct_select; heapless does not, so move the inherent methods
to Nct-only impl blocks, matching the already-Nct-gated Saturating* trait
forms. A branchless Ct saturating is left as future work, consistent with
heapless keeping its Ct trait surface minimal.

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