solver: select skyline candidate policy

Make candidate ordering an explicit deterministic policy and benchmark ascending, descending, and exact-width-first choices. Keep ascending as the default because it produces the best measured time to first solution despite best-fit's slightly smaller tree.

Retain the benchmark v1 interface and document why randomized and duplicate-work portfolio policies are deferred.

Tests: Release and Debug CTest (10 passed each)

Refs: #12

TESTING.md

@@ -12,14 +12,18 @@ 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 routed order-9 solver test
-checks that its search counters exactly match order 8.  The skyline tests 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.  That direct test is deliberately
-separate from the public order-9 route, which uses even-predecessor
-construction.  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 best-fit order-9 solver
+test checks that its search counters exactly match order 8.  The skyline tests
+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
+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
+selects odd construction.  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