feat(zfs): manage jeeves datasets declaratively from nix

Add a zfs_manager module that reconciles the live datasets on jeeves
against a nix declaration, and generate the snapshot retention config
from that same declaration so the two can no longer drift apart.

systems/jeeves/datasets.nix declares every dataset on the media, storage
and scratch pools, nested the way zfs nests them and flattened into
pool/parent/child names. Values were transcribed from the live pools
rather than from scripts/zfs.sh, which had gone stale: acltype reads back
as posix, and media/secure/important, scratch/kestra and storage/nomad
were never recorded. root_pool datasets are declared for retention only,
their properties stay unmanaged for now.

python.tools.zfs_manager creates missing datasets and corrects drifted
properties, and never destroys anything. Undeclared properties are judged
by the zfs source field, so inherited and default values stay quiet while
locally set ones warn. Size values are normalised to bytes so that 16K and
16384 do not re-issue zfs set on every run.

vars.nix now derives its paths from the declared mountpoints instead of
repeating them, dropping three zfs_* keys that nothing referenced.

Replaces systems/jeeves/snapshot_config.toml, which listed a dataset that
does not exist and omitted thirteen that do.

python/zfs/__init__.py

@@ -1,11 +1,23 @@
 """init."""
 
-from python.zfs.dataset import Dataset, Snapshot, get_datasets
+from python.zfs.dataset import (
+    Dataset,
+    Snapshot,
+    create_dataset,
+    get_datasets,
+    get_properties,
+    list_dataset_names,
+    set_property,
+)
 from python.zfs.zpool import Zpool
 
 __all__ = [
     "Dataset",
     "Snapshot",
     "Zpool",
+    "create_dataset",
     "get_datasets",
+    "get_properties",
+    "list_dataset_names",
+    "set_property",
 ]