fix(installer): harden disk selection and partitioning phase

The disk phase was the dominant source of incomplete installs. Six
concrete failure modes addressed in one pass:

1. Live-ISO USB excluded from the disk picker. select_disk previously
   filtered loop|ram|zram|sr but not the device the installer booted
   from; picking it would format the boot media mid-install. New
   detect_live_iso_devices walks /, /iso, /run/initramfs/live,
   /nix/.ro-store, /nix/store and resolves each backing device to its
   parent disk via lsblk -no PKNAME. Override with
   NOMARCHY_INSTALL_ALLOW_ISO_TARGET=1 for the developer case.

2. 10 GiB minimum-capacity preflight. Disko fails late and obscurely
   on undersized media; surface it while the picker is still open.

3. prewipe_target_drive rewritten:
   - Enumerates every active dm-crypt mapping via dmsetup ls and
     closes those whose backing device is on the target drive. The
     old version only knew about the hardcoded names "crypted" /
     "crypted_main" so an aborted multi-disk run or a non-Nomarchy
     install would leave a holder open and silently break the wipe.
   - Drops `|| true` from wipefs / sgdisk / dd. After the LUKS and
     swap teardown above, a real failure means something is still
     holding the device — surface that instead of papering over it.
   - udevadm settle bounded to 30s so a flapping USB can't hang.
   - Post-wipe sanity check: refuse to hand the disk to disko if
     anything is still mounted off it.

4. run_disko_with_retry wraps the disko call. On failure, shows the
   last 30 lines of output via gum style and offers Retry /
   View full log / Abort. set -e is suspended for the disko call so
   the exit code can be inspected. The previous bare `disko --mode
   disko` aborted the whole installer with output scrolled past.

5. Sed-templated disko-golden.nix + disko-btrfs-multi.nix pair
   replaced by a single disko-config.nix Nix function of
   { mainDrive, extraDrives ? [] } called via --argstr / --arg.
   Templating Nix via shell-escaped string substitution caused at
   least one production bug (3aadc36 fixed embedded-newline
   escaping); function arguments are the right shape and eliminate
   the entire class of escaping concerns. Single-disk path is
   `extraDrives = []`; multi-disk gets BTRFS `-d single -m raid1`
   plus the additional /dev/mapper/* devices. Hosts that shipped
   /etc/disko-golden.nix now ship /etc/disko-config.nix.

6. EXIT trap added so the tmpfs LUKS key file (/dev/shm/nomarchy-
   luks.key) is removed even if the script aborts between key-write
   and the explicit unset. Replaced redundant `shred -u` on tmpfs
   with `rm -f` (already in RAM).

Verification: bash -n on install.sh, nix-instantiate parse + strict
eval on disko-config.nix in both single and multi shapes, full
nix flake check --no-build evaluating all three NixOS configurations
(default, nomarchy-installer, nomarchy-live) plus the installerVm.

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

docs/STRUCTURE.md

@@ -125,9 +125,8 @@ The `lib/` directory provides centralized logic and data structures to maintain
 
 ### `installer/` (Bootstrap)
 - **`install.sh`**: The interactive TTY-based installer. It handles disk partitioning, NixOS installation, and generating a clean "Downstream" flake for the user.
-- **`disko-golden.nix`**: The standard partition layout (BTRFS on top of LUKS2).
-- **`disko-btrfs-multi.nix`**: Multi-disk BTRFS RAID/Single layout template.
-- **`disko-btrfs-luks.nix`**: A simpler reference layout for disk management.
+- **`disko-config.nix`**: The disko partition layout (BTRFS on top of LUKS2). A Nix function of `{ mainDrive, extraDrives ? [] }` — single-disk path is `extraDrives = []`; multi-disk adds BTRFS `-d single -m raid1` across the extras. Invoked by `install.sh` via `disko --argstr mainDrive … --arg extraDrives '[…]'`.
+- **`disko-btrfs-luks.nix`**: A simpler reference layout for disk management (not used by the installer).
 
 ### `hosts/` (Targets)
 - **`nomarchy-installer.nix`**: Configuration for the minimal, TTY-based installation ISO.