feat(installer): add multi-disk BTRFS support

- Allow selecting multiple drives in the TTY installer using gum choose --no-limit.
- Add installer/disko-btrfs-multi.nix template for BTRFS RAID/Single setups.
- Dynamically generate multi-disk disko configurations with LUKS-on-every-disk.
- Default to BTRFS 'single' data and 'raid1' metadata for maximum capacity across mismatched drives (e.g., 20GB + 120GB SSDs).
- Update roadmap and structure documentation to reflect the new capabilities.

docs/ROADMAP.md

@@ -78,7 +78,7 @@ Each PR description should reference the row(s) in `docs/SCRIPTS.md` it closes,
 - Software-profile multi-select (Now).
 - Richer disk metadata (Shipped).
 - Form-factor → laptop preset (Now, depends on Pillar 5).
-- `disko-golden.nix` variants for software-RAID and BTRFS-pool-as-root.
+- `disko-golden.nix` variants for software-RAID and BTRFS-pool-as-root (Shipped).
 - Pre-flight resume polish (Next).
 - "What's installed?" summary screen on boot of a freshly-installed system, sourced from `state.json` + `nomarchy-system-scripts` introspection.
 - Optional non-LUKS branch in the installer for users who explicitly opt out of FDE.
@@ -138,6 +138,7 @@ Nomarchy is moving away from being a "flavor" of Omarchy to its own distinct ide
 
 (Move items here when they land — keep them brief, link the commit/PR.)
 
+- _2026-04-26_ — Multi-disk BTRFS support in the installer. Added `installer/disko-btrfs-multi.nix` template and updated `installer/install.sh` to allow selecting multiple drives via `gum choose --no-limit`. Implements BTRFS "single" data + RAID1 metadata across multiple LUKS-encrypted drives.
 - _2026-04-26_ — Distro Branding Phase 2. Updated bootloader entries to use "Nomarchy" as the label. Set ISO volume IDs to `NOMARCHY_INSTALLER` and `NOMARCHY_LIVE`. Fixed branding in Plymouth theme metadata and SDDM metadata.
 - _2026-04-26_ — Distro Branding Phase 1. Renamed `installerIso` to `nomarchy-installer` and `installerIsoGraphical` to `nomarchy-live`. Updated metadata and host configurations. Scrubbed "Omarchy" from Plymouth and installer messages.
 - _2026-04-26_ — Fix `hardware-db.sh` missing in `nomarchy-installer.nix`. Resolved boot error where `install.sh` failed to source the hardware database on the TTY installer ISO.

docs/STRUCTURE.md

@@ -126,6 +126,7 @@ 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.
 
 ### `hosts/` (Targets)