fix(commands): restore child.register handler (alias registerChild)

The P6.3 refactor dropped the case 'registerChild' branch from the
nodeClass switch. Restored as a canonical child.register descriptor
with the legacy registerChild alias.

Monster accepts measurement children — without this, inbound Port 2
handshakes from a child were silently ignored.

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

src/commands/handlers.js

@@ -40,3 +40,15 @@ exports.setMode = (source, msg) => {
 exports.setModelPrediction = (source, msg) => {
   if (typeof source.setModelPrediction === 'function') source.setModelPrediction(msg.payload);
 };
+
+// Inbound child registration from a measurement (or other) child node.
+// Ported from the legacy `case 'registerChild'` branch in nodeClass.
+exports.childRegister = (source, msg, ctx) => {
+  const childId = msg.payload;
+  const childObj = ctx?.RED?.nodes?.getNode?.(childId);
+  if (!childObj?.source) {
+    (ctx?.logger || source.logger)?.warn?.(`child.register skipped: missing child/source for id=${childId}`);
+    return;
+  }
+  source.childRegistrationUtils.registerChild(childObj.source, msg.positionVsParent, msg.distance);
+};

src/commands/index.js

@@ -44,4 +44,10 @@ module.exports = [
     payloadSchema: { type: 'any' },
     handler: handlers.setModelPrediction,
   },
+  {
+    topic: 'child.register',
+    aliases: ['registerChild'],
+    payloadSchema: { type: 'string' },
+    handler: handlers.childRegister,
+  },
 ];