solver: construct odd-order solutions

Avoid repeating the exponential search for odd orders at least nine. Search the even predecessor, translate its row-major placements to the enlarged board, and tile the new border.

Keep direct search and construction explicit so benchmarks can report their costs separately. Verify the routed order-9 result independently and require its search counters to match order 8.

Tests: Release, Debug, ASan and UBSan CTest (8 passed each)

Refs: #6

TESTING.md

@@ -12,8 +12,10 @@ ctest --test-dir build --output-on-failure
 The tests independently check board dimensions, square multiplicities, bounds,
 overlap, and complete coverage.  They cover small unsatisfiable solver inputs, a
 known order-8 solution, invalid placement diagnostics, and construction of an
-order-9 solution from the order-8 fixture.  The rendering test also formats the
-known order-8 solution and checks the resulting grid dimensions and coverage.
+order-9 solution from the order-8 fixture.  The routed order-9 solver test
+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.
 
 ## Debug and sanitizers