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

TESTING.md

@@ -17,6 +17,11 @@ checks that its search counters exactly match order 8.  The rendering test also
 formats the known order-8 solution and checks the resulting grid dimensions and
 coverage.
 
+When Python is available, `reference-support` also tests the dependency-free
+placement JSON validator.  If the optional OR-Tools package is present, it
+additionally builds and solves the trivial order-1 CP-SAT model; otherwise that
+part is skipped without changing the default test result.
+
 ## Debug and sanitizers
 
 Use a separate build directory for each configuration: