games/openenroth: introduce some minor improvements and fixes

• Reflect currently required C++ standard (23) in the USES line
• Convert GH_ACCOUNT/GH_PROJECT/GH_TAGNAME/GH_SUBDIR lists into GH_TUPLE which is shorter, more blame-friendly and thus easier to maintain (assess changes)
• Depend on `math/fast_float' implementation from the ports
• Try to unbreak the build against Clang 21.1.8 by initializing const pointer and explicitly invoking operator bool() when asserting unique_ptr

Reported by: pkg-fallout