Hide `VMap` constructor

Giving ability to construct `VMap` with underlying `KVVector`, allows to bypass internal invariant and is unsafe.

Also make sure `unVMap` cannot be used as field modifier.

Locate the VMap type and its exported constructor, then inspect how unVMap is exposed and used as a field modifier. Confirm the existing invariant around the underlying KVVector and identify relevant tests or module export checks. Done means callers cannot construct VMap directly or use unVMap as a field modifier, while existing valid usage remains covered.

Written by the indexing model from the issue text.