refactor(snapshot): Add explicit type ignores for seal methods

why: The snapshot classes need to implement seal methods to be compatible
     with the SealableProtocol, but these methods are added dynamically
     by the frozen_dataclass_sealable decorator at runtime.

what:
- Add proper type ignores for all seal methods with attr-defined to silence
  mypy errors about methods not defined in the superclass
- Improve module docstring to explain type checking nuances with property
  overrides and seal methods
- Fix import order and general code style
- Ensure consistent docstrings for properties
- Add explicit body to seal methods so they're properly overriding the
  decorator-provided implementation

refs: This works in conjunction with the mypy override in pyproject.toml

Author: tony