feat(dashboardapi): diff-skip regen via flows:started predicate (#36)

Subscribes to Node-RED's flows:started runtime event, caches the {diff}
payload on the dashboardAPI source, and short-circuits the child.register
handler when none of {dashboardAPI id, child id, grandchild ids} appears in
diff.added/changed/removed/rewired. Predicate verified by S1 spike (#32).

- src/nodeClass.js: _attachLifecycleHook subscribes, _attachCloseHandler
  cleans up. No-op when RED.events isn't available (unit-test friendly).
- src/specificClass.js: subtreeChanged() predicate + subtreeIdsFor() helper.
- src/commands/handlers.js: registerChild consults predicate before composing;
  logs {event:'regen-skipped', outcome:'no-diff'} when skipping.
- test/basic/slice36-diff-predicate.basic.test.js: 8 cases — null/empty diff,
  affected/unaffected ids, tab-id over-triggering avoidance, grandchild
  inclusion, no-grandchild case.

Cold start (no cached diff yet) always regenerates — safe default. Edge case
documented in #32: when a brand-new child is wired to a registered parent,
the new child's id is in diff.added but not yet in registeredChildren when
flows:started fires. Mitigation (b) per spike findings: one-deploy race
accepted for R&D — next deploy picks up the new registration.

Closes #36

src/commands/handlers.js

@@ -24,12 +24,33 @@ function resolveChildNode(childId, ctx) {
 
 // On child.register: build the dashboard graph (root + direct children) and
 // emit one Grafana upsert HTTP request per dashboard on Port 0.
+//
+// Diff-skip behavior (PRD F-1, S1 spike #32): if the latest flows:started
+// payload's `diff` indicates that NEITHER the dashboardAPI itself NOR this
+// child NOR its grandchildren changed, skip composition and log no-diff. The
+// first call after startup (no cached diff yet) regenerates unconditionally.
 function registerChild(source, msg, ctx) {
   const childSource = resolveChildSource(msg.payload, ctx);
   if (!childSource?.config) {
     throw new Error('Missing or invalid child node');
   }
 
+  const subtreeIds = source.subtreeIdsFor(ctx.node?.id, childSource);
+  const changed = source.subtreeChanged(source.lastFlowsStartedDiff, subtreeIds);
+  if (!changed) {
+    if (source.logger?.info) {
+      source.logger.info({
+        event: 'regen-skipped',
+        outcome: 'no-diff',
+        trigger: 'child.register',
+        dashboardApiId: ctx.node?.id,
+        childId: childSource?.config?.general?.id,
+        subtreeSize: subtreeIds.size,
+      });
+    }
+    return;
+  }
+
   const dashboards = source.generateDashboardsForGraph(childSource, {
     includeChildren: Boolean(msg.includeChildren ?? true),
   });