Add staged inter-distance propagation with variable separation

[zayenz]

Add inter-distance constraints enforcing |x[i] - x[j]| >= p for a constant or variable separation.

IPL_BASIC uses an O(n log n), linear-space feasibility algorithm and tightens a variable separation's upper bound. The other propagation levels add quadratic bounds(Z) filtering. Separation one uses the existing bounds-consistent distinct propagator; domains with holes use interval supports.

The implementation follows Garey et al., Quimper et al., and the variable-distance extension. Existing Gecode tests check feasibility and supported bounds against an independent tuple oracle. Sanitizer runs and cross-platform CI passed.

[zayenz]

• Combined filter preparation and pruning. The sole caller no longer has to perform a separate initialization step.
• Documented the dual links encoding and the adjustment cursor owned by each live bound-group representative.