Define VEntry and virtual filesystem (VFS) core types

[DamianReeves]

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.