fix: node graph edge display, teardown crashes, and event/audio lifetime bugs

- capi: oakengine_node_output_connection_at/_at_ex returned the source
  node as the connection destination; the actual destination is
  conn.second.node(). Out-edge enumeration was useless, so the node
  view could only draw in-edges and randomly lost whichever edges
  needed the out-edge path (random per context build order).
  Regression test in oakengine_node_test
- project teardown: Project::clear() pre-notifies node removal while
  nodes are fully constructed (observers used to crash on
  half-destroyed nodes); childEvent suppresses the removal dance while
  clearing; Node::disconnect_all/disconnect_edge get a silent mode for
  teardown so no invalidation/events touch dying members
  (is_being_cleared); ClipBlock marker disconnect guarded against
  dead viewer/markers; ProjectCopier and PreviewAutoCacher drop
  project references on Project::destroyed instead of disconnecting
  dead objects at shutdown
- preview: add oakengine_preview_request_get_audio_sample_count; the
  viewer queried sample count by passing nullptr to get_audio_samples
  which rejects it, so all playback audio was silently dropped
- app: fix unterminated input-id memcpy in ResolveGroupInput
  (nodeparamviewitem, widgetbridge) that corrupted every parameter id
- app: unsubscribe raw C-API event subscriptions in destructors of
  NodeParamViewKeyframeControl, NodeParamViewConnectedLabel and
  ExportDialog; playhead events used to fire into dead widgets
  (crash when dragging the playhead)
- tests: preview request roundtrip (video frame + audio range) and
  free-while-active teardown coverage; env-gated OAK_DEBUG_EDGES /
  OAK_DEBUG_INVALID_INPUT diagnostics
- docs: investigation notes in docs/zh/

engine/node/project.cpp

@@ -86,6 +86,23 @@ void Project::initialize()
 
 void Project::clear()
 {
+	is_being_cleared_ = true;
+
+	// Notify observers about each node while it is still fully constructed.
+	// The childEvent() removal path fires from ~Node, when derived-class
+	// members are already destroyed; observers touching the node there
+	// (e.g. querying a Sequence's tracks) used to crash.
+	const QVector<Node *> notify_copy = node_children_;
+	for (Node *node : notify_copy) {
+		if (!node_children_.contains(node)) {
+			// An observer already removed it from the graph
+			continue;
+		}
+		emit node_removed(node);
+		emit node->removed_from_graph(this);
+		node->RemovedFromGraphEvent(this);
+	}
+
 	// By deleting the last nodes first, we assume that nodes that are most important are deleted last
 	// (e.g. Project's ColorManager or ProjectSettingsNode.
 	for (auto it = node_children_.cbegin(); it != node_children_.cend(); it++) {
@@ -95,6 +112,8 @@ void Project::clear()
 	while (!node_children_.isEmpty()) {
 		delete node_children_.last();
 	}
+
+	is_being_cleared_ = false;
 }
 
 SerializedData Project::load(QXmlStreamReader *reader)
@@ -369,6 +388,14 @@ void Project::childEvent(QChildEvent *event)
 		} else if (event->type() == QEvent::ChildRemoved) {
 			node_children_.removeOne(node);
 
+			if (is_being_cleared_) {
+				// Teardown: everything is being destroyed anyway. The
+				// disconnect/emit dance below touches the half-destroyed
+				// child (this fires from ~Node) and has repeatedly crashed;
+				// Qt auto-disconnects the rest when destruction completes.
+				return;
+			}
+
 			// Disconnect signals
 			disconnect(node, &Node::input_connected, this,
 					   &Project::input_connected);