update flake.lock; refactor impermanence; other minor fixes

hosts/imxyy-nix-x16/hardware.nix

@@ -30,13 +30,12 @@ in
     };
   };
 
-  fileSystems."/" = {
+  my.persist.btrfs = {
     device = btrfs;
-    fsType = "btrfs";
-    options = [
-      "compress=zstd"
-      "subvol=root"
-    ];
+    mountPoint = "/nix/persist";
+    persistSubvol = "persist";
+    rootSubvol = "root";
+    zstdCompress = true;
   };
 
   fileSystems."/nix" = {
@@ -48,42 +47,6 @@ in
     ];
   };
 
-  my.persist.location = "/nix/persist";
-  fileSystems."/nix/persist" = {
-    device = btrfs;
-    fsType = "btrfs";
-    options = [
-      "compress=zstd"
-      "subvol=persist"
-    ];
-    neededForBoot = true;
-  };
-
-  boot.initrd.postDeviceCommands = lib.mkAfter ''
-    mkdir /btrfs_tmp
-    mount ${btrfs} /btrfs_tmp
-    mkdir -p /btrfs_tmp/old_roots
-    if [[ -e /btrfs_tmp/root ]]; then
-        timestamp=$(date --date="@$(stat -c %Y /btrfs_tmp/root)" "+%Y-%m-%-d_%H:%M:%S")
-        mv /btrfs_tmp/root "/btrfs_tmp/old_roots/$timestamp"
-    fi
-
-    delete_subvolume_recursively() {
-        IFS=$'\n'
-        for i in $(btrfs subvolume list -o "$1" | cut -f 9- -d ' '); do
-            delete_subvolume_recursively "/btrfs_tmp/$i"
-        done
-        btrfs subvolume delete "$1"
-    }
-
-    for i in $(find /btrfs_tmp/old_roots/ -maxdepth 1 -mtime +14); do
-        delete_subvolume_recursively "$i"
-    done
-
-    btrfs subvolume create /btrfs_tmp/root
-    umount /btrfs_tmp
-  '';
-
   fileSystems."/boot" = {
     device = "/dev/disk/by-uuid/96D3-93B0";
     fsType = "vfat";