refactor(nixos): organize host modules

configs/nixos/modules/host/zfs.nix

@@ -0,0 +1,62 @@
+{
+  config,
+  lib,
+  ...
+}:
+# The host-level ZFS pool import: durable service state a host mounts, never rebuilds.
+let
+  cfg = config.modules.host.zfs; # NOLINT(let_order)
+in
+{
+  options.modules.host.zfs = {
+    enable = lib.mkEnableOption "importing durable ZFS pools that hold service state";
+
+    pools = lib.mkOption {
+      type = lib.types.attrsOf (lib.types.attrsOf lib.types.path);
+      default = { };
+      example = lib.literalExpression ''
+        {
+          tank = {
+            media = "/srv/media";
+            downloads = "/srv/downloads";
+          };
+        }
+      '';
+      description = ''
+        The ZFS pools to import at boot, keyed by pool name, each pool mapping a
+        dataset path relative to it to that dataset's mountpoint. A pool is
+        durable state imported as it stands, never created or destroyed by a
+        rebuild, so a service's data survives any rebuild or reimage. A pool
+        with an empty map is still imported, leaving each dataset to its own ZFS
+        `mountpoint` property.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # The ZFS stack in the kernel and boot, needed even where the root filesystem is another kind.
+    boot.supportedFilesystems = [ "zfs" ];
+
+    # Derive a stable 32-bit host ID from the configured hostname.
+    networking.hostId = builtins.substring 0 8 (
+      builtins.hashString "sha256" config.networking.hostName
+    );
+
+    # The declared pools are imported at boot, distinct from any pool backing the root filesystem.
+    boot.zfs.extraPools = lib.attrNames cfg.pools;
+
+    # Each declared dataset is mounted at its host path as a native ZFS filesystem.
+    fileSystems = lib.mkMerge (
+      lib.mapAttrsToList (
+        pool: mounts:
+        lib.mapAttrs' (
+          dataset: mountpoint:
+          lib.nameValuePair mountpoint {
+            device = "${pool}/${dataset}";
+            fsType = "zfs";
+          }
+        ) mounts
+      ) cfg.pools
+    );
+  };
+}