introduce nexus templating via nib

cerulean/nexus/default.nix

@@ -0,0 +1,20 @@
+# Copyright 2025 Emile Clark-Boman
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+{mix, ...} @ inputs:
+mix.newMixture inputs (mixture: {
+  includes.public = [
+    ./nodes.nix
+    ./nexus.nix
+  ];
+})