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

[zayenz]

Adds integer GCD, reified divisibility, exact n-ary products, and fixed/variable-modulus products. All reified forms support equivalence and both implication modes. GCD is nonnegative, zero divides zero, and modular products use Euclidean residues.

Propagation uses bounds, signs, powers, congruence, and algebraic rewrites without tuple enumeration. The public documentation states the propagation limits and fixed/variable modulus semantics. Includes the 6.5.0 release note and a modeling example.

Readiness review fixes include sparse-domain fixpoint rescheduling, avoiding repeated space allocations for result aliases, and cheap reified identities. Tests cover signed limits, overflowing exact products with known modular residues, activation, cloning, and variable-to-fixed modulus rewrites. They are registered in Gecode's normal testing framework, with no family-specific CI or make check additions.

Validation: all 591 integer arithmetic tests pass locally in Release and Debug with runtime auditing and UBSan (halt on error). A separately configured installed-package consumer compiled and executed all nine overloads. The sanitizer run also exposed and fixed two existing arithmetic test-oracle overflows. Integrated current main; supported-platform CI is running on the final revision.

A small reproducible scaling sample records the tradeoff between n-ary propagation cost and improved search for repeated factors and result aliases. No speculative caching or broad performance rewrite is included.

See docs/gcd-pr-readiness.md for the review and follow-up evidence.