add a `ZENGINE_VALIDATE_ASSERT(m_meta.size() < 64, ...)` inside `Register()` to enforce the documented 64-slot capacity constraint at runtime

[JeanPhilippeKernel]

All three items from the previous review are addressed. One non-blocking suggestion: add a ZENGINE_VALIDATE_ASSERT(m_meta.size() < 64, ...) inside Register() to enforce the documented 64-slot capacity constraint at runtime rather than relying solely on the comment. Fine to do as a follow-up. Ready to merge.

Originally posted by @JeanPhilippeKernel in #662 (review)