kaidokert / fixed-bigint-rs / 29695007441

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Display/hex/FromStr/Num for HeaplessBigInt (string surface) (#153)

* Add Display/LowerHex/UpperHex/FromStr/Num to HeaplessBigInt

The string/parse surface, Nct-only, over the value width (len) — the last
num_traits::PrimInt blocker (Num / from_str_radix).

- Display / LowerHex / UpperHex: feature-independent. Digit extraction goes
  through the MachineWord const_num_traits::ToPrimitive supertrait (like
  FixedUInt's to_radix_str), not num_traits, so a heapless bignum is printable
  without opting into num-traits. Hex mirrors FixedUInt (MSB-first, leading-
  zero suppression; zero renders empty, Display special-cases it to "0").
- Num (from_str_radix) / FromStr: gated behind num-traits, reusing FixedUInt's
  ParseIntError constructors (now pub(crate)).
- Inherent div_rem exposed on the Nct carrier (closes a parity gap; Display
  uses it).

Width subtlety: from_str_radix accumulates at CAP width (ret widened up front),
not the minimal width of the intermediate from(digit) values — otherwise
ret*radix+digit would overflow at a single word instead of the carrier's
capacity. That matches FixedUInt<T, CAP>, whose parse width is its N.

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