fix: settler icon colour + CONTRACT child.register row

- settler.html node colour was #e4a363 (legacy orange); aligned to
  S88 Unit blue #50a8d9 per .claude/rules/node-red-flow-layout.md §16
  cleanup list. Existing flows are unaffected — colour is editor-only.
- CONTRACT.md: add `child.register` row; previously mentioned only in
  prose ("not listed here"), but contract-verify enforces the table.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

settler.html

@@ -3,7 +3,7 @@
 <script type="text/javascript">
   RED.nodes.registerType("settler", {
     category: "EVOLV",
-    color: "#e4a363",
+    color: "#50a8d9",
     defaults: {
       name: { value: "" },
       processOutputFormat: { value: "process" },