switch subvolume_list parser to --format json once it's out of experimental

[danneu]

Context

btrfs subvolume list supports --format json, but it's gated behind #if EXPERIMENTAL in btrfs-progs (cmds/subvolume-list.c:1747). Standard nixpkgs builds don't pass --enable-experimental, so the JSON flag is unavailable.

Our parse_btrfs_subvolume_list nom text parser works but is fragile — any change to btrfs-progs text formatting could break it silently.

What to do

When btrfs-progs promotes subvolume list --format json out of experimental:

1. Switch BtrfsSubvolumeList in cmd.rs to pass --format json
2. Rewrite cli/src/parse/btrfs_subvolume_list.rs from nom to serde_json
3. Follow the same pattern used for the device stats JSON migration

JSON schema (from current experimental source)

{
  "__header": { "version": "1" },
  "subvolume-list": [
    {
      "id": 256,
      "generation": 30,
      "top_level": 5,
      "path": "data"
    }
  ]
}

Field names from cmds/subvolume-list.c rowspec: id (u64), generation (u64), top_level (u64), path (string). Additional fields appear when CLI flags like -u, -p, -g are used.

Tracking

Watch btrfs-progs releases for the #if EXPERIMENTAL guard being removed from subvolume list.