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>

CONTRACT.md

@@ -8,10 +8,9 @@ Hand-maintained for Phase 6; the `## Inputs` table is generated from
 | Canonical | Aliases (deprecated) | Payload | Effect |
 |---|---|---|---|
 | `data.influent` | `influent`, `setInfluent` | `{ F: number, C: number[13] }` — either field optional | Replaces influent flow and/or the 13-species concentration vector. Triggers `output-changed`, re-emits the 3-stream Fluent envelope. |
+| `child.register` | `registerChild` | `string` — the child node's Node-RED id | Resolves the child via `RED.nodes.getNode` and registers it through `childRegistrationUtils` at the supplied `msg.positionVsParent`. |
 
 Aliases log a one-time deprecation warning the first time they fire.
-Plumbing topics (`child.register`) are handled by the BaseNodeAdapter and
-not listed here.
 
 ## Outputs