VariantConstructors

Caches the engine's per-type Variant constructor functions.

get_variant_from_type_constructor / ..._to_type_constructor are relatively expensive lookups, but the function pointer they return is stable for the lifetime of the process. gdext caches these in a table indexed by variant type; we do the same.

Variant type tags are small contiguous integers (NIL=0 .. MAX), so a flat array indexed by the tag is the natural cache. The cache is populated lazily on first use of each type and never invalidated.