Add protected namespace for custom nodes to store sensitive data
(API keys, licenses) that cannot be accessed via HTTP endpoints.
Key changes:
- New API: get_system_user_directory() for internal access
- New API: get_public_user_directory() with structural blocking
- 3-layer defense: header validation, path blocking, creation prevention
- 54 tests covering security, edge cases, and backward compatibility
System Users use `__` prefix (e.g., __system, __cache) following
Python's private member convention. They exist in user_directory/
but are completely blocked from /userdata HTTP endpoints.
