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. Other propagation levels add staged 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.

Use the standard Gecode integer test harness for assignment, search, cloning, and consistency checks. Larger tests alternate arbitrary assignments with valid separated values constructed in sorted order with bounded total slack, then shuffled. Small hole, residue, and limit examples exhaustively enumerate their individual domains. Posting exception checks use the existing test space; no custom model remains.

Based on main after #249 merged. #253 remains stacked on this PR.

Validation: Release build passed; all 74 inter-distance tests passed with five iterations. The updated #253 stack also passed the combined selection of these tests and 12 fixed-base power tests. Whitespace checks 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.