Define VEntry and virtual filesystem (VFS) core types

Background:
- Pipeline design requires VEntry and VFS abstractions to represent inputs/outputs.

Scope:
- Implement VEntry interface and core entry types (file, folder, document, node, archive) with metadata.
- Implement VFS mounts with precedence and RO/RW zones.

Acceptance criteria:
- VEntry and VFS types are defined with basic traversal and metadata support.
- Mount precedence and RO/RW enforcement are test-covered.