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/nodeundo.cpp

@@ -134,6 +134,12 @@ NodeEdgeAddCommand::~NodeEdgeAddCommand()
 
 void NodeEdgeAddCommand::redo()
 {
+	if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+		qWarning("EDGE-DEBUG: NodeEdgeAddCommand::redo %p -> %p (%s)",
+				 (void *)output_, (void *)input_.node(),
+				 qPrintable(input_.input()));
+	}
+
 	if (input_.is_connected()) {
 		if (!remove_command_) {
 			remove_command_ =
@@ -148,6 +154,12 @@ void NodeEdgeAddCommand::redo()
 
 void NodeEdgeAddCommand::undo()
 {
+	if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+		qWarning("EDGE-DEBUG: NodeEdgeAddCommand::undo %p -> %p (%s)",
+				 (void *)output_, (void *)input_.node(),
+				 qPrintable(input_.input()));
+	}
+
 	Node::disconnect_edge(output_, input_);
 
 	if (remove_command_) {
@@ -169,11 +181,23 @@ NodeEdgeRemoveCommand::NodeEdgeRemoveCommand(Node *output,
 
 void NodeEdgeRemoveCommand::redo()
 {
+	if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+		qWarning("EDGE-DEBUG: NodeEdgeRemoveCommand::redo %p -> %p (%s)",
+				 (void *)output_, (void *)input_.node(),
+				 qPrintable(input_.input()));
+	}
+
 	Node::disconnect_edge(output_, input_);
 }
 
 void NodeEdgeRemoveCommand::undo()
 {
+	if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+		qWarning("EDGE-DEBUG: NodeEdgeRemoveCommand::undo %p -> %p (%s)",
+				 (void *)output_, (void *)input_.node(),
+				 qPrintable(input_.input()));
+	}
+
 	Node::connect_edge(output_, input_);
 }