Represent partial placements as column heights and branch on the narrowest local valley. This removes the board-area cell state and makes first-solution search substantially smaller for feasible orders.
Expose direct-search and candidate-order benchmark controls so the skyline core can be measured independently of odd-order construction. Document the completeness argument and the 10/11 test-tier decision.
Tests: Release, Debug, ASan, and UBSan CTest (10 passed each)
Refs: #4
Codex instance
d751d1b13e