venn returns its three groups as a named record

## Summary

Give `zfs.replicate.list.venn` a return type that names its three groups, so
callers reach them by name rather than by position.

## Motivation

`venn` returns `tuple[list[T], list[T], list[T]]` (`zfs/replicate/list.py:22`).
All three elements have the same type, so nothing but the docstring maps a
position to a meaning: left-only, common, right-only.

The single caller unpacks it positionally and routes the groups to opposite
actions:

```python
lefts, middles, rights = venn(local_snaps, remote_snapshots[destination])

send_tasks = _send_snapshots(remote, lefts)
destroy_tasks = _destroy_snapshots(destination, rights)
```

Transposing `lefts` and `rights` there would generate destroy tasks for
snapshots that should be sent and send tasks for ones that should be destroyed.
mypy cannot catch it, because it sees three interchangeable `list[T]`.

Fowler's Replace Primitive with Object is the fix. Surfaced while reviewing #501
through the refactoring catalogue and declined there to keep that change to
annotation churn.

## Scope

- Add a `NamedTuple` to `zfs/replicate/list.py` whose fields carry the
docstring's vocabulary for the three groups.
- Return it from `venn`.
- Read the groups by name at `zfs/replicate/task/generate.py:39`.
- Update the three `venn` property tests in
`zfs_test/replicate_test/list_test.py`, which destructure the result.

## Acceptance criteria

- [ ] Transposing two groups at a call site is a type error rather than a
silent behaviour change.
- [ ] `task/generate.py` reads the groups by name.
- [ ] The docstring no longer has to explain what each component position means.

## Additional context

Martin Fowler, _Refactoring_ 2nd edition: Replace Primitive with Object.

Start with zfs/replicate/list.py:22 and inspect the caller at zfs/replicate/task/generate.py:39. Run the three venn property tests in zfs_test/replicate_test/list_test.py before and after the change. Done means the result exposes named groups, the caller reads those names, and positional group transposition is rejected by typing.

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