kaidokert / fixed-bigint-rs / 29714086842

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Add CtCheckedAdd/Sub/Mul to HeaplessBigInt (#170)

* Complete the HeaplessBigInt CT surface: CtChecked* and Ct NextPowerOfTwo

The last remaining CT traits — none actually blocked, just unbuilt.

- CtCheckedAdd/Sub/Mul (arith.rs): Overflowing* + subtle::CtOption::new — the
  overflow flag masks observability instead of branching. P-generic, no
  ct_select needed (same shape as CtNonZero).
- NextPowerOfTwo now impls the Ct carrier too (power_of_two.rs): the Nct arm
  branches (panic / wrap), the Ct arm is branchless via ct_select (saturate to
  the width-max for `next`, wrap to zero for `wrapping`). Both share a
  P-generic value-based `checked_next_pow2` helper; `checked_next_power_of_two`
  itself stays branchful (Option leaks the overflow bit) — not CT, same caveat
  as FixedUInt, so Ct callers use next/wrapping.

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