Add minimum-distance constraints with single and decomposed propagation

[zayenz]

Add exact minimum-distance constraints over selected site indices: z = min(d(x[i], x[j])). Distances use a shared validated matrix or a shared callable evaluated on demand, and optional inclusive requirements apply to selected-position pairs.

Forward propagation is available as MDP_SINGLE, the default advisor-based organization with cached pair maxima, or MDP_DECOMPOSED, with one actor per pair and a finalizer. The single organization avoids the quadratic actor count. Optional arguments place ipl before the organization. A separate greedy conflict-matching upper bound is enabled by default and disabled with IPL_BASIC. Propagation is intentionally incomplete; repeated sites and variable aliases are supported.

The algorithms follow M. Z. Lagerkvist, Propagation Algorithms for the Minimum-Distance Constraint over Selected Points, ModRef 2026. See the paper implementation and paper discussion and evidence. Public documentation and the changelog credit the paper.

Validation:

• All 42 focused minimum-distance tests passed twice, covering both organizations, matrix/callable distances, pair requirements, aliases, cloning, zero witnesses, matching certificates and shared callback lifetime.
• Release build and configured check target passed; the configured check was rerun after the independent readiness review.
• Independent zdev branch review found no substantive defects. Changelog tidy and git diff --check passed.
• Local performance tests compared the organizations and distance representations. Benchmarks and result artifacts are excluded from this branch.
• Cross-platform CI remains to be checked. Windows, Autotools and sanitizer execution were not verified locally.