[zayenz]

Add experimental A* and complete anytime beam search (CABS) for ordinary Gecode Space subclasses. Stacked on #248.

Both engines use a bound on the total objective to prune search. An optional ranking function guides exploration without affecting pruning. A* selects pending spaces by bound or rank; CABS keeps a beam at each depth and repeats with wider beams when discarded candidates could still improve the best solution. CABS also accepts model-defined dominance between candidates in the next layer.

Both engines support parallel search through Options::threads and dynamic worker control. Their builders can be combined with BAB in PBS portfolios. As with BAB, next() returns improving solutions; the astar(...) and cabs(...) helpers return the best solution found before exhaustion or a stop.

Fifteen-puzzle and TSPTW demonstrate the engines. TSPTW compares a closed CP model with a transition model that extends the route one city at a time, using BAB, A*, or CABS. It includes one small runnable input. Correctness tests use Gecode's native test suite.

Enable with GECODE_ENABLE_EXPERIMENTAL=ON or --enable-experimental; the preview is disabled by default. Preview APIs may change or be removed in future feature releases.

Validation: focused open-list, heuristic-search, preparation, and PBS tests passed under ASan/UBSan and with static mimalloc 3.5.3. Development checks also compared TSPTW readers, enumerated small optima, and checked published tours. Leak detection was disabled for existing test-registration and thread-pool lifetimes.