refactor(flake): add user registry and multi-user host support

Separate user identity (data) from the reusable modules, and let a host
declare any number of users instead of exactly one.

- users/registry.nix: per-user identity (name, email, groups, authorized
  and signing keys) as the single source of identity; no user data is
  hardcoded in the modules.
- mkHost takes a `users` set keyed by username; per-user identity is
  injected into each home config via the `identity` module arg
  (extraSpecialArgs is per-host, so it cannot carry per-user data).
- modules/users.nix builds accounts from the registry; modules/ssh.nix no
  longer defines authorized keys (the registry owns them); home/git.nix
  and home/desktop.nix read `identity`; users/emmathorpe/work.nix drops
  its now-redundant git identity override.
- Restructure the tree: users/, home/, modules/, hosts/, lib/ replace the
  former lyrathorpe/ and system/ layout.
- Add standalone homeConfigurations (the portable subset: shell, git,
  editor, claude) and an exported homeModules output for use on machines
  not managed by this flake, or as an input to other flakes.

Behaviour-preserving for existing hosts: lyrathorpe-mbp and
emmathorpe-edaas evaluate to identical derivations; lyrathorpe-t400,
lyrathorpe-macpro31 and lyrathorpe-rpi5 differ only by de-duplicating a
repeated authorized_keys entry. Fixes the SSH authorized-key leak (one
user's key was applied to every account), the hardcoded default git
identity, and the hardcoded EDaaS linger setting.

hosts/MBP-Asahi/configuration.nix

@@ -0,0 +1,46 @@
+# MacBook Pro (Apple Silicon, Asahi NixOS). Shared laptop options live in
+# ../../modules/laptop.nix; only host-specific settings are here.
+{ pkgs, ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+  ];
+
+  # UEFI boot via systemd-boot. Asahi manages the EFI vars from macOS, so do not
+  # touch them from NixOS.
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = false;
+
+  networking.hostName = "Lyra-Asahi";
+
+  # Audio (PipeWire) and the swaylock PAM stack are inherited from
+  # workstation.nix. hardware.enableRedistributableFirmware is also set there;
+  # it is harmless here since Asahi supplies its own peripheral firmware below.
+
+  # Binary cache for the Asahi kernel/build artifacts, so the MBP pulls prebuilt
+  # outputs instead of compiling the Asahi kernel locally.
+  nix.settings = {
+    substituters = [ "https://nixos-apple-silicon.cachix.org" ];
+    trusted-public-keys = [
+      "nixos-apple-silicon.cachix.org-1:8psDu5SA5dAD7qA0zMy5UT292TxeEPzIz8VVEr2Js20="
+    ];
+  };
+
+  # Apple peripheral firmware (Wi-Fi/Bluetooth). The directory is gitignored and
+  # populated out-of-band -- see README.
+  hardware.asahi.peripheralFirmwareDirectory = ../../modules/firmware;
+
+  environment.systemPackages = with pkgs; [
+    asahi-bless
+    asahi-nvram
+    asahi-btsync
+    asahi-wifisync
+    unzip
+    ppp
+    iptables
+  ];
+
+  # See `man configuration.nix` / the stateVersion docs before changing.
+  system.stateVersion = "25.05";
+}