Add darling-lol --set-prefix to keep the prefix on another drive

The prefix holds the installed clients and game (about 40 GB). The Nix
launcher always exported DPREFIX=${DPREFIX:-~/.darling}, so the prefix saved
in build/config.json was never used and the only way to move it was an
environment variable in every launch context.

- The launcher uses DPREFIX if set, then the saved prefix, then ~/.darling.
- darling-lol --set-prefix PATH runs scripts/nix-prefix.py in the namespace,
  whose root owns the prefix files, so cp -a keeps their owners and xattrs.
  It renames the prefix when possible; across filesystems it copies to
  PATH.partial and removes the old prefix and its overlay work directory
  only after the copy completes. A PATH that already holds a Darling prefix
  is used as it is. It refuses non-empty directories that are not prefixes,
  nested paths and paths with commas or colons (overlayfs separators), then
  saves the path with configure.py --prefix.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/NIX.md

@@ -210,8 +210,15 @@ Darling namespace; `darling-lol --stop` can still stop it.
 ## Files and operational limits
 
 - Workspace: `${XDG_DATA_HOME:-$HOME/.local/share}/darling-lol`.
-- Default guest prefix: `$HOME/.darling`; set `DPREFIX` to use another prefix.
-  Explicit `DPREFIX` and `RIOT_GUEST_HOME` overrides take precedence over saved
+- Default guest prefix: `$HOME/.darling`. The prefix holds the installed
+  clients and game, about 40 GB. To keep it on another drive, close the clients
+  and run `darling-lol --set-prefix /mnt/games/league`. This moves an existing
+  prefix there and saves the path for later launches. Across filesystems it
+  copies first and removes the old prefix only after the copy completes. If the
+  path already holds a Darling prefix, it switches to it and moves nothing. The
+  path must not contain commas or colons (overlayfs uses them as separators).
+  The next launch sets up the runtime in the new prefix.
+- Explicit `DPREFIX` and `RIOT_GUEST_HOME` overrides take precedence over saved
   configuration, so clear stale overrides before switching installations.
 - Package upgrades refresh managed tools and remove previously tracked obsolete
   files. Builds, the installed runtime, guest prefixes, and unmanaged files are