feat: separate Node class into FileNode and DirNode classes

[Copilot]

This PR separates the monolithic Node class into specialized FileNode and DirNode classes that inherit from a common abstract Node base class, improving code organization and type safety.

Changes Made

• Abstract Node base class: Contains common functionality like timestamps, permissions, mode, uid/gid, symlink support, and permission checks
• FileNode class: Extends Node with file-specific operations (getString(), setString(), getBuffer(), setBuffer(), write(), read(), truncate(), and the buf property)
• DirNode class: Extends Node for directories (no buffer operations needed)
• Smart node creation: Updated Superblock.createNode() to instantiate the appropriate node type based on file mode (S_IFDIR → DirNode, otherwise → FileNode)
• Updated exports: Added FileNode and DirNode to module exports alongside the base Node class

Benefits

• Better separation of concerns: File-specific operations are now isolated to FileNode
• Type safety: Abstract base class prevents direct instantiation of the generic Node class
• Backward compatibility: All existing APIs continue to work exactly as before
• Memory efficiency: Directory nodes no longer carry unnecessary buffer-related properties

Testing

• All existing tests pass without modification (except updating the node-specific test to use FileNode)
• Manual verification confirms both node types work correctly with their respective operations
• Build and linter checks pass successfully

The implementation maintains full backward compatibility while providing a cleaner, more maintainable codebase structure.

Fixes #1181.

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