solver: break board dihedral symmetry

Constrain the unique unit square to a closed D4 fundamental region once its placement is known. This preserves one representative of every board-orientation orbit without assigning identities to repeated squares.

Keep a symmetry-disabled benchmark path, document the proof and measurements, and cover generic, diagonal, midline, corner, and centre orbits.

Tests: Release, Debug, ASan, and UBSan CTest (11 passed each)

Refs: #3

TESTING.md

@@ -18,7 +18,11 @@ check smallest-width valley selection and deterministic tie-breaking, validate
 an order-8 result with descending candidates, and independently validate a
 direct order-9 search with ascending candidates.  An
 exhaustive infeasible order also checks that all three policies visit the same
-search space.  The direct order-9 test is deliberately separate from the
+search space.  Focused D4 tests cover all eight rotations and reflections,
+even and odd midlines, diagonals, corners, and the odd-board centre.  They
+independently validate order-8 solutions with symmetry enabled and disabled
+and check that only the enabled search records symmetry pruning.  The direct
+order-9 test is deliberately separate from the
 public order-9 route, which uses even-predecessor construction.  Orders 10 and
 11 are not routine tests because both public routes require the same long
 direct order-10 search; the route-boundary test still checks that order 11