Add Euclidean division APIs and coverage

[Copilot]

The paper now includes Euclidean quotient/remainder operations, but this repo did not yet implement them. This change adds the Euclidean APIs and covers the key correctness property: the remainder is always non-negative and strictly less than abs(y).

• API surface

  • Add div_rem_euclid(x, y) for Euclidean quotient/remainder pairs
  • Add div_euclid(x, y) for quotient-only access
  • Add rem_euclid(x, y) as a dedicated direct remainder computation instead of projecting from div_rem_euclid

• Implementation

  • Implement Euclidean adjustment from truncating division while preserving the invariant:
    • x == y * q + r
    • 0 <= r < abs(y) for signed inputs
  • Keep unsigned behavior aligned with ordinary division, since the remainder is already Euclidean there
  • Avoid extra Euclidean recomputation in the remainder-only path

• Test coverage

  • Add Euclidean validation helpers alongside the existing rounding-mode checks
  • Add targeted static_assert cases for sign combinations of dividend/divisor
  • Extend fuzz coverage for both int and unsigned to verify:
    • quotient/remainder identity holds
    • div_euclid / rem_euclid match div_rem_euclid
    • Euclidean remainder stays in range

static_assert(div_rem_euclid(-8,  3) == div_result<int>{-3, 1});
static_assert(div_rem_euclid(-8, -3) == div_result<int>{ 3, 1});
static_assert(div_rem_euclid( 8, -3) == div_result<int>{-2, 2});

static_assert(div_euclid(-8, 3) == -3);
static_assert(rem_euclid(8, -3) == 2);