fix(installer): harden multi-disk LUKS, password handling, revision pinning

Several installer reliability fixes that were left uncommitted:

- Impermanence + multi-disk LUKS: disko-config.nix names the main LUKS
  mapping `crypted` for single-disk and `crypted_main` once extraDrives is
  non-empty. The impermanence rollback hook used to hardcode `crypted`,
  which made every multi-disk install fail to mount root in initrd. Added
  a `nomarchy.system.impermanence.mainLuksName` option and wired the
  installer to write the correct value into the generated system.nix
  based on the drive count.

- Password no longer cleartext in /etc/nixos: installer now hashes the
  user password with `mkpasswd -m sha-512` and emits
  `initialHashedPassword` instead of `initialPassword`. Added mkpasswd to
  the live ISO. Cleartext is unset immediately after hashing.
  USER_PASSWORD_HASH is deliberately not persisted in --resume state —
  configure_user re-prompts on resume.

- Revision pinning that actually works on the live ISO: `inputs.self`
  strips .git in the Nix store copy, so `git rev-parse HEAD` would silently
  return empty on a real install and the generated flake would track main.
  Live ISO now writes `/etc/nomarchy-rev` from `inputs.self.rev` at build
  time; install.sh reads it first, falls back to git, and aborts with a
  loud confirmation prompt if both are empty (instead of silently
  installing an unpinned system).

- Generated `/mnt/etc/nixos/state.json`: toggle scripts (nomarchy-tz-select,
  nomarchy-setup-{fido2,fingerprint}, nomarchy-toggle-hybrid-gpu,
  nomarchy-wifi-powersave) `jq` this file in place and fail hard if it
  doesn't exist. Fresh installs now ship a schema-conformant file matching
  lib/state-schema.nix.

- Unmount /mnt before exiting `finish()` regardless of reboot choice. Clean
  unmount avoids dirty BTRFS on reboot; on "no", leaving /mnt mounted
  blocked a second installer run on the same live ISO.

- Removed obsolete `installer/disko-btrfs-luks.nix` (superseded by
  `disko-config.nix` per commit 3aadc36) and dropped its dangling
  `docs/STRUCTURE.md` reference.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/system/impermanence.nix

@@ -10,13 +10,27 @@ in
 
   options.nomarchy.system.impermanence = {
     enable = lib.mkEnableOption "Erase Your Darlings (Impermanence) root wipe on boot";
+
+    # The disko layout names the main LUKS mapping `crypted` on single-disk
+    # installs and `crypted_main` on multi-disk installs (see
+    # installer/disko-config.nix: `mainLuksName`). The rollback hook must
+    # mount the right device, otherwise initrd fails on every boot and the
+    # @ → root-blank snapshot is never restored.
+    mainLuksName = lib.mkOption {
+      type = lib.types.str;
+      default = "crypted";
+      description = ''
+        Name of the /dev/mapper entry holding the BTRFS root. Set to
+        "crypted_main" on multi-disk installs to match the disko layout.
+      '';
+    };
   };
 
   config = lib.mkIf cfg.enable {
     # 1. The Rollback Script: Runs in initrd before filesystems are mounted
     boot.initrd.postDeviceCommands = lib.mkAfter ''
       mkdir -p /btrfs_tmp
-      mount -o subvol=/ /dev/mapper/crypted /btrfs_tmp
+      mount -o subvol=/ /dev/mapper/${cfg.mainLuksName} /btrfs_tmp
       
       if [[ -e /btrfs_tmp/@ ]]; then
           mkdir -p /btrfs_tmp/old_roots