Fixed: boolean operations failed on Blender 5.x.

- The add-on asked the Boolean modifier for a solver named FAST. Blender has replaced that set with FLOAT, EXACT and MANIFOLD, so the assignment raised and the operation stopped there — Quick Union, cut and difference all failed with the same internal message.

- The solver is now chosen from what the running Blender actually offers, so the add-on works on old and new versions alike.

Verified: two blocks union into one on Blender 5.1, and the exported level comes out as expected.

Nothing else changed. Upgrading is safe: remove the old version in Preferences, install this one, restart Blender.