Add integer GCD, divisibility, and n-ary product constraints

[zayenz]

Add integer GCD, reified divisibility, exact n-ary products, and products with fixed or variable moduli. Reification supports equivalence and both implication modes. GCD is nonnegative, zero divides zero, and modular products use Euclidean residues.

Propagation uses bounds, signs, congruences, and algebraic rewrites without tuple enumeration. Public API comments describe the propagation limits and modulus semantics. Small products reuse existing equality and multiplication propagators where appropriate.

The existing arithmetic suite passes in Release and audited Debug with UBSan. Installed-package checks cover the public overloads.

[zayenz]

• Corrected the gcd_abs_status() comment. The function checks interior domain membership as well as bounds.
• Reused multiple_bounds() across GCD, divisibility, and modular-product propagation. This removes duplicate rounding and bound updates; the divisibility caller normalizes the sign.

[zayenz]

No actionable findings in GCD, divisibility, or product propagation. All 663 registered arithmetic tests passed locally, and all 14 CI checks pass at 87e8d23b0e.