Expose missing native propagators through MiniZinc and FlatZinc

[zayenz]

The Gecode MiniZinc library exposes missing native propagators through standard fzn_ hooks and additional modelling predicates in gecode.mzn.

The standard hooks cover count inequalities, conditionals, nondeterministic regular constraints, optional distinctness and scheduling, and set intersection. MiniZinc retains responsibility for public overloads, assertions, index handling and optional values. Builtin expressions also use native implementations for extrema, supported float powers, set matrix element and scalar implication.

The Gecode-specific predicates provide inter-distance and minimum distance, arithmetic constraints including array-wide and modular products, offset distinctness and matching counts, optional rectangle non-overlap, and additional array, channeling and set relations. These interfaces reach native propagators that standard MiniZinc formulations do not reach, or avoid auxiliary variables through views.

Public predicates lower to typed bindings in gecode_fzn.mzn, with documented semantics and reification support. Propagation strength is selected through annotations. Fixed-base floating-point powers and inverse logarithms use explicit MPFR rounding.

Validation includes all 135 FlatZinc regressions, 131 MiniZinc integration cases, and predicate documentation generation.

Fixed-value conditional hooks delegate to their variable-value counterparts. The regular posters use concrete integer arguments, Boolean extrema preserve shared inputs directly, and the annotation test checks the exact propagation flags. The conditional library files follow modern upstream MiniZinc formatting.

The simplifications passed a release build, all 135 FlatZinc regressions and 16 shared-Boolean extrema tests with five iterations, 16 conditional compilation comparisons and executions, and four direct shared-Boolean registry cases.