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

Same fix as monster: P6.6 refactor dropped the case 'registerChild'
branch when extracting commands. Settler registers reactor + measurement
children — without this, Port 2 inbound handshakes were silently
ignored.

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

src/commands/handlers.js

@@ -5,11 +5,11 @@
 //   msg:    the Node-RED input message.
 //   ctx:    { node, RED, send, logger } — provided by BaseNodeAdapter.
 //
-// Settler currently accepts no behavioural commands — the legacy
-// `registerChild` topic is handled by the BaseNodeAdapter input dispatch
-// via the generalFunctions registry (`child.register` canonical) and the
-// node never had a public set/cmd surface beyond that. Future
-// influent-injection or operator-override topics will land here.
+// Settler accepts `child.register` (alias `registerChild`) on Port 0 input
+// to register measurement / reactor children, plus `data.influent` for
+// manual override. BaseNodeAdapter dispatches msg.topic through the
+// per-node registry — there is no implicit `child.register` handler in
+// the base, so it must be listed explicitly here.
 
 function _logger(source, ctx) {
   return ctx?.logger || source?.logger || null;
@@ -30,3 +30,16 @@ exports.dataInfluent = (source, msg, ctx) => {
   if (Array.isArray(p.C)) source.Cs_in = [...p.C];
   source.notifyOutputChanged();
 };
+
+// Inbound child registration from a measurement (or reactor) child.
+// Ported from the legacy `case 'registerChild'` branch in nodeClass.
+exports.childRegister = (source, msg, ctx) => {
+  const log = _logger(source, ctx);
+  const childId = msg.payload;
+  const childObj = ctx?.RED?.nodes?.getNode?.(childId);
+  if (!childObj?.source) {
+    log?.warn?.(`child.register skipped: missing child/source for id=${childId}`);
+    return;
+  }
+  source.childRegistrationUtils.registerChild(childObj.source, msg.positionVsParent, msg.distance);
+};

src/commands/index.js

@@ -14,4 +14,10 @@ module.exports = [
     payloadSchema: { type: 'any' },
     handler: handlers.dataInfluent,
   },
+  {
+    topic: 'child.register',
+    aliases: ['registerChild'],
+    payloadSchema: { type: 'string' },
+    handler: handlers.childRegister,
+  },
 ];