[zayenz]

Adds a read-only Space::compare() that ranks two solutions as better, equivalent, worse, or incomparable. Every space used for optimization must now implement both constrain() and compare(). constrain() still posts the improvement restriction; compare() decides which of two solutions to keep. The default compare() throws SpaceNoComparison.

Engines:

• Sequential BAB and RBS: when an external solution arrives through constrain, compare() decides whether it replaces the incumbent.
• Parallel BAB and PBS portfolios: every solution is compared with the retained incumbent. Only SC_BETTER replaces it, so equivalent and worse solutions are dropped. If compare() throws, the exception reaches the caller, and the engine has to be deleted.
• A* and CABS (experimental): compare() decides which solutions become the incumbent. The guidance bound still orders and prunes open nodes.
• All engines reject incomparable solutions with Search::Incomparable.

Models and helpers:

• Gecode::Sense (minimize, maximize) is a shared optimization direction. compare_int(x, y, sense) compares two integer objective values, so an integer compare() is one line, like constrain().
• The experimental HeuristicGuidance now uses Gecode::Sense instead of its own MINIMIZE/MAXIMIZE enum.
• The MiniModel integer, lexicographic and float optimization spaces, FlatZinc and car-sequencing implement compare(). Float spaces compare the bound their cuts use, ignoring the step.

Tests:

• Comparison tests cover the engines, portfolios, external incumbents, failure transport, FlatZinc and compare_int.
• RBS and worker-control tests had relied on BAB enumerating equal solutions, which no longer happens. They now check strict improvement.
• Static counters shared across tests caused races under the concurrent harness. Each test now has its own counter.

Verified locally on macOS: search and FlatZinc tests pass in both the default and the experimental build, and the doc target builds.

Before this leaves draft, .zdev/ will be dropped from the branch.

[zayenz]

• The one-argument CarSequencing::compare() hid the inherited Gist display overload. Added using Script::compare to restore the two-argument call.
• Parallel PBS broadcast a solution before handling a stored worker failure. That could call into a failed asset and replace the original exception. The failure guard now runs before solution processing.