[Core]: Fix entity generation overflow

[nircoe]

entities::destroy() incremented a recycled id's generation with no bound, letting it wrap past entity::MAX_GENERATIONS and risk colliding with the invalid entity sentinel.
It now retires an id permanently once its generation reaches the maximum, instead of recycling it.

• Guard destroy() against generation overflow, retiring an id once it reaches entity::MAX_GENERATIONS
• Remove the redundant generation check in create()'s recycle path
• Add a regression test covering the recycle path up to and past the generation limit