Declare the disk layout with disko

Installing used to mean six manual steps through Calamares, one of which
copied hardware-configuration.nix into the repo — a step that fails
silently unless the file is also git added, since Nix only sees tracked
files. It also did not scale past one machine.

modules/disk.nix defines moonarch.disk.{enable,device,encrypt,espSize,
swapSize} and builds disko.devices from it, so another machine is a
device path rather than a new layout. Installing is a single
disko-install run from the ISO.

hardware-configuration.nix is no longer needed: nvme, ahci, sd_mod and
the USB HID modules are in the initrd via
boot.initrd.includeDefaultModules, and nixpkgs.hostPlatform comes from
nixosSystem. The testvm copy described a qcow2 that no longer exists and
is deleted; that host now uses the module against /dev/vda.

The user password no longer arrives with the installer, so it is set
once via nixos-enter after installing instead of committing a hash.

modules/disk.nix

@@ -0,0 +1,127 @@
+# ABOUTME: Declarative disk layout for every host: GPT, optional LUKS2, btrfs.
+# ABOUTME: A host sets moonarch.disk.device; the layout itself is shared.
+
+{ config, lib, ... }:
+
+let
+  cfg = config.moonarch.disk;
+
+  mountOptions = [ "compress=zstd" "noatime" ];
+
+  subvolumes = {
+    "/root" = { mountpoint = "/"; inherit mountOptions; };
+    "/home" = { mountpoint = "/home"; inherit mountOptions; };
+    # Separate so the store stays out of the snapper snapshots of /. It is
+    # reproducible from the flake and would only inflate them.
+    "/nix" = { mountpoint = "/nix"; inherit mountOptions; };
+  } // lib.optionalAttrs (cfg.swapSize != "") {
+    # No compression or noatime here: disko creates the swapfile inside this
+    # subvolume, and btrfs requires it to be nodatacow, which it sets itself.
+    "/swap" = {
+      mountpoint = "/.swapvol";
+      swap.swapfile.size = cfg.swapSize;
+    };
+  };
+
+  btrfs = {
+    type = "btrfs";
+    # Overwrite an existing signature instead of asking. The device is erased
+    # by this point either way.
+    extraArgs = [ "-f" ];
+    inherit subvolumes;
+  };
+
+  # Neither passwordFile nor settings.keyFile is set, which makes disko's
+  # askPassword default to true: it prompts for the passphrase twice while
+  # formatting and never puts it on a command line.
+  luks = {
+    type = "luks";
+    name = "crypted";
+    # Lets TRIM reach the SSD through the container. The tradeoff is that the
+    # pattern of used blocks becomes visible on the raw device.
+    settings.allowDiscards = true;
+    content = btrfs;
+  };
+in
+{
+  options.moonarch.disk = {
+    enable = lib.mkEnableOption "the declarative disk layout";
+
+    device = lib.mkOption {
+      type = lib.types.str;
+      example = "/dev/disk/by-id/nvme-eui.0123456789abcdef";
+      description = ''
+        Whole disk to partition, as a /dev/disk/by-id/ path — that one survives
+        enumeration changes, unlike /dev/nvme0n1.
+
+        This value decides which disk gets erased. Check it against lsblk on the
+        target machine before installing.
+
+        Only the partitioning reads it. The generated fileSystems entries go
+        through /dev/disk/by-partlabel/, so a booted system does not depend on
+        the path staying the same.
+      '';
+    };
+
+    encrypt = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = ''
+        Put the filesystem inside a LUKS2 container. The layout is otherwise
+        identical, so a machine that does not need encryption — a throwaway VM —
+        gets the same subvolumes.
+      '';
+    };
+
+    espSize = lib.mkOption {
+      type = lib.types.str;
+      default = "1G";
+      description = ''
+        Size of the EFI system partition. Every boot entry keeps its kernel and
+        initrd here, so this scales with the number of generations kept.
+      '';
+    };
+
+    swapSize = lib.mkOption {
+      type = lib.types.str;
+      default = "8G";
+      example = "";
+      description = ''
+        Size of the swapfile in the /.swapvol subvolume. Empty string creates no
+        swap at all.
+
+        Sized as a safety valve, not as a replacement for RAM. Hibernation needs
+        at least as much swap as RAM plus a resume_offset kernel parameter, which
+        is not set up here — see DECISIONS.md.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    disko.devices.disk.main = {
+      type = "disk";
+      device = cfg.device;
+      content = {
+        type = "gpt";
+        partitions = {
+          ESP = {
+            size = cfg.espSize;
+            type = "EF00";
+            content = {
+              type = "filesystem";
+              format = "vfat";
+              mountpoint = "/boot";
+              mountOptions = [ "umask=0077" ];
+            };
+          };
+          # Named the same whether or not it is encrypted, so the partlabel the
+          # fileSystems entries point at does not depend on cfg.encrypt.
+          root = {
+            size = "100%";
+            content = if cfg.encrypt then luks else btrfs;
+          };
+        };
+      };
+    };
+  };
+}