fix(CONTRACT): add child.register row — was in registry but not the table

The commands registry has `child.register` (with `registerChild` alias);
CONTRACT.md mentioned it only in the Port 2 prose. contract-verify
required it in the canonical Inputs table.

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

CONTRACT.md

@@ -13,6 +13,7 @@ Hand-maintained for Phase 6; the `## Inputs` table is generated from
 | `data.flow` | `input_q` | `{ value: number, unit: string }` | Converts to m³/h and pushes into `flow.manual.atequipment`. Blends with measured-child flow in `getEffectiveFlow()`. |
 | `set.mode` | `setMode` | string | Delegated to `source.setMode()` if defined. Reserved for future use. |
 | `set.model-prediction` | `model_prediction` | numeric | Delegated to `source.setModelPrediction()` if defined. Reserved for future use. |
+| `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.