The AS_OBJ macro on NaN-tagged architectures was performing a direct cast from
uint64_t to Obj* which triggers a pointer-to-integer cast warning on 32-bit
MinGW where sizeof(Obj*) is 4 bytes. Adding an intermediate cast to uintptr_t
ensures the value is properly truncated before conversion to the pointer type.
Also replaces IS_SINGLETON macro usage with direct NULL_VAL/UNDEFINED_VAL
comparisons and removes the now-unused IS_SINGLETON definition.