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

CMakeLists.txt

@@ -29,5 +29,8 @@ if(BUILD_TESTING)
     add_test(NAME benchmark-format
       COMMAND "${Python3_EXECUTABLE}" "${CMAKE_CURRENT_SOURCE_DIR}/benchmarks/run.py"
               --self-test)
+    add_test(NAME reference-support
+      COMMAND "${Python3_EXECUTABLE}"
+              "${CMAKE_CURRENT_SOURCE_DIR}/tests/reference_test.py")
   endif()
 endif()