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.

installer/disko-btrfs-multi.nix

@@ -0,0 +1,76 @@
+{
+  disko.devices = {
+    disk = {
+      main = {
+        type = "disk";
+        device = "@MAIN_DRIVE@";
+        content = {
+          type = "gpt";
+          partitions = {
+            ESP = {
+              priority = 1;
+              name = "ESP";
+              start = "1M";
+              end = "1G";
+              type = "EF00";
+              content = {
+                type = "filesystem";
+                format = "vfat";
+                mountpoint = "/boot";
+                mountOptions = [ "umask=0077" ];
+              };
+            };
+            luks = {
+              size = "100%";
+              content = {
+                type = "luks";
+                name = "crypted_main";
+                settings = {
+                  allowDiscards = true;
+                  passwordFile = "/dev/shm/nomarchy-luks.key";
+                };
+                content = {
+                  type = "btrfs";
+                  extraArgs = [ "-f" "-d single" "-m raid1" @BTRFS_DEVICES@ ];
+                  subvolumes = {
+                    "@" = {
+                      mountpoint = "/";
+                      mountOptions = [ "compress=zstd" "noatime" ];
+                    };
+                    "@persist" = {
+                      mountpoint = "/persist";
+                      mountOptions = [ "compress=zstd" "noatime" ];
+                    };
+                    "@home" = {
+                      mountpoint = "/home";
+                      mountOptions = [ "compress=zstd" "noatime" ];
+                    };
+                    "@nix" = {
+                      mountpoint = "/nix";
+                      mountOptions = [ "compress=zstd" "noatime" ];
+                    };
+                    "@log" = {
+                      mountpoint = "/var/log";
+                      mountOptions = [ "compress=zstd" "noatime" ];
+                    };
+                    "@snapshots" = {
+                      mountpoint = "/.snapshots";
+                      mountOptions = [ "compress=zstd" "noatime" ];
+                    };
+                  };
+                  postCreateHook = ''
+                    MNTPOINT=$(mktemp -d)
+                    mount -t btrfs /dev/mapper/crypted_main $MNTPOINT
+                    btrfs subvolume snapshot -r $MNTPOINT/@ $MNTPOINT/root-blank
+                    umount $MNTPOINT
+                  '';
+                };
+              };
+            };
+          };
+        };
+      };
+      @ADDITIONAL_DISKS@
+    };
+  };
+}