Document and address known gaps in _future() method

[jtdub]

Description

The _future() method in base.py:272-281 has a docstring that documents known unhandled cases:

def _future(self, config, future_config) -> None:
    """The below cases still need to be accounted for:
    - negate a numbered ACL when removing an item
    - idempotent command avoid list
    - and likely others.
    """

The method is also flagged with # noqa: C901 for complexity. These gaps are documented in code but not tracked anywhere visible to users or contributors.

Proposed Improvement

1. Track each known gap as a sub-task or checklist item (this issue can serve as the tracker)
2. Add a note in the future-config.md docs page about known limitations
3. Consider reducing _future() complexity by extracting the distinct cases (sectional overwrite, idempotent, negation, etc.) into helper methods

Known Gaps Checklist

• Negate a numbered ACL when removing an item
• Idempotent command avoid list handling
• Audit for additional unhandled edge cases