Add NixOS module with library deployment support

[csingley]

Summary

• Factors the NixOS module out of flake.nix into nix/module.nix, adds nix/lib.nix with a typed path schema DSL
• New module options: srcRoot, librariesDir, stashDir, autoDeploy, libraries
• The libraries attrset is keyed by library name (subdirectory under librariesDir); each entry specifies an optional src corpus subpath prefix (defaults to "" = entire corpus) and a pathSchema
• Path schemas are typed lists of tag references and string literals via music-magic.lib.tags, giving Nix-level tag name validation rather than untyped template strings
• Also includes: lofty 0.23.2 → 0.24 upgrade, transcode stash location fix, git commit guidance in CLAUDE.md

Example configuration

{ inputs, pkgs, ... }:
let inherit (inputs.music-magic.lib) tags; in {
  services.music-magic = {
    enable       = true;
    srcRoot      = "/srv/media/audio/inbox";
    librariesDir = "/srv/media/audio";
    stashDir     = "/srv/media/audio/stash";
    autoDeploy   = true;

    libraries."music" = {
      pathSchema = with tags; [ ARTIST ALBUM [ TRACKNUMBER " - " TITLE ] ];
    };
  };
}

Multi-library example:

libraries = {
  "music" = {
    pathSchema = with tags; [ ARTIST ALBUM [ TRACKNUMBER " - " TITLE ] ];
  };
  "audiophile" = {
    src        = "vinyl-rips";
    pathSchema = with tags; [ ARTIST ALBUM [ TRACKNUMBER " - " TITLE ] ];
  };
};

Test plan

• nixos-rebuild switch with the example configuration
• Verify config.kdl and dirs.kdl are generated correctly in /var/lib/mm/
• Verify corpus deploys to librariesDir/<library-name>/ with correct path schema
• Verify autoDeploy triggers deployment on corpus changes