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/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)