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/nodeClass.js

@@ -26,6 +26,29 @@ class nodeClass {
 
     this._attachInputHandler();
     this._attachCloseHandler();
+    this._attachLifecycleHook();
+  }
+
+  // Subscribe to Node-RED's `flows:started` event to cache the deploy diff so
+  // the child.register handler can decide whether *this* dashboardAPI's
+  // subtree was affected. Predicate documented in Gitea issue #32 spike.
+  _attachLifecycleHook() {
+    if (!this.RED?.events?.on) return;
+    this._flowsStartedListener = (payload) => {
+      const diff = payload?.diff || null;
+      this.source.lastFlowsStartedDiff = diff;
+      this.source.lastFlowsStartedAt = Date.now();
+      if (this.source?.logger?.debug) {
+        const summary = diff
+          ? Object.fromEntries(
+              ['added', 'changed', 'removed', 'rewired', 'linked', 'flowChanged']
+                .map((k) => [k, (diff[k] || []).length])
+            )
+          : null;
+        this.source.logger.debug({ event: 'flows:started', type: payload?.type, diff: summary });
+      }
+    };
+    this.RED.events.on('flows:started', this._flowsStartedListener);
   }
 
   _buildConfig(uiConfig) {
@@ -78,6 +101,10 @@ class nodeClass {
 
   _attachCloseHandler() {
     this.node.on('close', (done) => {
+      if (this._flowsStartedListener && this.RED?.events?.off) {
+        this.RED.events.off('flows:started', this._flowsStartedListener);
+        this._flowsStartedListener = null;
+      }
       if (typeof done === 'function') done();
     });
   }