feat(lib): add stable service uid registry

lib/service-identities.nix

@@ -0,0 +1,59 @@
+{ lib }:
+let
+  defaultIdentities = {
+    actualbudget = 20001;
+    copyparty = 20002;
+  };
+
+  duplicateUidMessage = duplicates: ''
+    Duplicate stable service UID allocation(s): ${lib.concatStringsSep ", " (
+      map (entry: "${toString entry.uid} is assigned to ${lib.concatStringsSep ", " entry.names}") duplicates
+    )}
+  '';
+
+  mk = identities:
+    let
+      names = lib.attrNames identities;
+      namesByUid = lib.foldl' (
+        acc: name:
+        let
+          uid = toString identities.${name};
+        in
+        acc // {
+          ${uid} = (acc.${uid} or [ ]) ++ [ name ];
+        }
+      ) { } names;
+      duplicates = lib.mapAttrsToList (uid: namesForUid: {
+        uid = lib.toInt uid;
+        names = namesForUid;
+      }) (lib.filterAttrs (_uid: namesForUid: builtins.length namesForUid > 1) namesByUid);
+      checkedIdentities =
+        assert lib.assertMsg (duplicates == [ ]) (duplicateUidMessage duplicates);
+        identities;
+    in
+    rec {
+      inherit checkedIdentities;
+
+      serviceUid = name:
+        if builtins.hasAttr name checkedIdentities then
+          checkedIdentities.${name}
+        else
+          throw "Unknown stable service identity `${name}`. Known identities: ${lib.concatStringsSep ", " (lib.attrNames checkedIdentities)}";
+    };
+
+  default = mk defaultIdentities;
+
+  tests = {
+    actualbudgetUid = default.serviceUid "actualbudget" == 20001;
+    copypartyUid = default.serviceUid "copyparty" == 20002;
+    missingLookupFails = !(builtins.tryEval (default.serviceUid "missing-service")).success;
+    duplicateUidFails = !(builtins.tryEval (mk {
+      first = 20003;
+      second = 20003;
+    }).checkedIdentities).success;
+  };
+in
+default // {
+  identities = default.checkedIdentities;
+  inherit tests;
+}