bench: add optional CP-SAT reference

Provide a separately installed OR-Tools model to compare a generic constraint solver with the native first-solution path without adding a production or default-test dependency.

Use no-overlap, exact-fill, edge and equal-copy symmetry constraints, validate placements independently, and record model size, memory, worker count and timings for orders 8 and 9. Keep the tool only as a reference and defer DLX absent new evidence.

Tests: Release CTest (9 passed)

Tests: Python reference tests and compilation checks

Tests: independently validated CP-SAT orders 8 and 9

Refs: #5

.gitignore

@@ -1 +1,5 @@
 cmake-build*/
+build*/
+.venv*/
+__pycache__/
+*.pyc