fix(impermanence): user must match created account, not hardcoded "nomarchy"

The persistence block at core/system/impermanence.nix:75 read
`users.nomarchy = { directories = [...]; }` — the username was a
literal, not a reference. For any user not literally named "nomarchy"
the block was silently inert and ~/.ssh, ~/.gnupg, ~/.local/share/keyrings,
Documents, Downloads, Pictures, Videos, Projects were wiped on every boot.

Adds `nomarchy.system.impermanence.user` (str, default "nomarchy") and
uses it via `users.${cfg.user}`. The installer now writes the chosen
username alongside `enable` and `mainLuksName` so impermanence installs
with non-default usernames are correct out of the box.

docs/OPTIONS.md: fixes the wrong path on the impermanence row
(documented `impermanence.enable`, real option is
`nomarchy.system.impermanence.enable`) and adds entries for
`mainLuksName` and `user`.

Found during Pillar 8 audit of core/system modules.

installer/install.sh

@@ -1459,7 +1459,7 @@ generate_flake_config() {
         if (( ${#_drives[@]} > 1 )); then
             _main_luks_name="crypted_main"
         fi
-        impermanence_opt=$'nomarchy.system.impermanence.enable = true;\n  nomarchy.system.impermanence.mainLuksName = "'"$_main_luks_name"$'";'
+        impermanence_opt=$'nomarchy.system.impermanence.enable = true;\n  nomarchy.system.impermanence.mainLuksName = "'"$_main_luks_name"$'";\n  nomarchy.system.impermanence.user = "'"$USERNAME"$'";'
     fi
 
     local PROFILE_SYSTEM_OPTS=""