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/

app/dialog/export/export.cpp

@@ -462,6 +462,16 @@ ExportDialog::ExportDialog(OakEngineNode *viewer_node, bool stills_only_mode,
 	subtitle_tab_->setEnabled(subtitles_enabled_->isChecked());
 }
 
+ExportDialog::~ExportDialog()
+{
+	// Raw C-API subscription carries `this` as userdata; not covered by
+	// Qt's auto-disconnect.
+	if (viewer_sub_ > 0) {
+		oakengine_event_unsubscribe(viewer_sub_);
+		viewer_sub_ = 0;
+	}
+}
+
 Rational ExportDialog::get_selected_timebase() const
 {
 	return video_tab_->get_selected_frame_rate().flipped();

app/dialog/export/export.h

@@ -49,6 +49,7 @@ public:
 		: ExportDialog(viewer_node, false, parent)
 	{
 	}
+	~ExportDialog() override;
 
 	Rational get_selected_timebase() const;
 	void set_selected_timebase(const Rational &r);

app/widget/nodeparamview/nodeparamviewconnectedlabel.cpp

@@ -110,6 +110,14 @@ NodeParamViewConnectedLabel::NodeParamViewConnectedLabel(const oak::Input &input
 			&NodeParamViewConnectedLabel::set_value_tree_visible);
 }
 
+NodeParamViewConnectedLabel::~NodeParamViewConnectedLabel()
+{
+	// Raw C-API subscription carries `this` as userdata; not covered by
+	// Qt's auto-disconnect. Unsubscribe or the next playhead event calls
+	// into a dead object.
+	set_viewer_node(nullptr);
+}
+
 void NodeParamViewConnectedLabel::set_viewer_node(OakEngineNode *viewer)
 {
 	if (viewer_) {

app/widget/nodeparamview/nodeparamviewconnectedlabel.h

@@ -37,6 +37,7 @@ class NodeParamViewConnectedLabel : public QWidget {
 public:
 	NodeParamViewConnectedLabel(const oak::Input &input,
 								QWidget *parent = nullptr);
+	~NodeParamViewConnectedLabel() override;
 
 	void set_viewer_node(OakEngineNode *viewer);
 

app/widget/nodeparamview/nodeparamviewitem.cpp

@@ -37,8 +37,9 @@ static oak::Input ResolveGroupInput(const oak::Input &input)
 	char input_id[256];
 	int element = input.element();
 	const QByteArray utf = input.input_id().toUtf8();
-	memcpy(input_id, utf.constData(), qMin<int>(sizeof(input_id) - 1, utf.size()));
-	input_id[sizeof(input_id) - 1] = '\0';
+	const int len = qMin<int>(sizeof(input_id) - 1, utf.size());
+	memcpy(input_id, utf.constData(), len);
+	input_id[len] = '\0';
 	// WRAPPER-GAP: oakengine_group_resolve_input (group API has no oak:: wrapper)
 	if (oakengine_group_resolve_input(
 			node, input_id, element,

app/widget/nodeparamview/nodeparamviewkeyframecontrol.cpp

@@ -157,6 +157,20 @@ NodeParamViewKeyframeControl::NodeParamViewKeyframeControl(bool right_align,
 	show_buttons_from_keyframe_enable(false);
 }
 
+NodeParamViewKeyframeControl::~NodeParamViewKeyframeControl()
+{
+	// Raw C-API subscription carries `this` as userdata; it is not covered
+	// by Qt's auto-disconnect. Without this, a playhead event delivered
+	// after destruction calls update_state() on a dead object.
+	if (viewer_sub_ > 0) {
+		oakengine_event_unsubscribe(viewer_sub_);
+		viewer_sub_ = 0;
+	}
+	// Drop the keyframe_* bridge subscriptions too (same raw-userdata
+	// mechanism underneath).
+	set_input(oak::Input());
+}
+
 void NodeParamViewKeyframeControl::set_input(const oak::Input &input)
 {
 	if (input_.is_valid()) {

app/widget/nodeparamview/nodeparamviewkeyframecontrol.h

@@ -41,6 +41,7 @@ public:
 		: NodeParamViewKeyframeControl(true, parent)
 	{
 	}
+	~NodeParamViewKeyframeControl() override;
 
 	const oak::Input &get_connected_input() const
 	{

app/widget/nodeparamview/nodeparamviewwidgetbridge.cpp

@@ -142,9 +142,9 @@ static bool ResolveGroupInput(oak::Input *input)
 	char input_id[256];
 	int element = input->element();
 	const QByteArray utf = input->input_id().toUtf8();
-	memcpy(input_id, utf.constData(),
-		   qMin<int>(sizeof(input_id) - 1, utf.size()));
-	input_id[sizeof(input_id) - 1] = '\0';
+	const int len = qMin<int>(sizeof(input_id) - 1, utf.size());
+	memcpy(input_id, utf.constData(), len);
+	input_id[len] = '\0';
 	if (!oakengine_node_group_get_inner(&node, input_id, sizeof(input_id),
 									&element)) {
 		return false;

app/widget/nodeview/nodeviewcontext.cpp

@@ -255,6 +255,12 @@ bool NodeViewContext::child_input_disconnected(OakEngineNode *output,
 	for (int i = 0; i < edges_.size(); i++) {
 		NodeViewEdge *e = edges_.at(i);
 		if (e->output() == oak::Node(output) && e->input() == input) {
+			if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+				qWarning("EDGE-DEBUG: edge removed: %p -> %p (%s,%d) ctx=%p",
+						 (void *)output, (void *)input.node_handle(),
+						 qPrintable(input.input_id()), input.element(),
+						 (void *)this);
+			}
 			delete e;
 			edges_.removeAt(i);
 			return true;
@@ -413,6 +419,11 @@ void NodeViewContext::mousePressEvent(QGraphicsSceneMouseEvent *event)
 
 void NodeViewContext::add_node_internal(OakEngineNode *node, NodeViewItem *item)
 {
+	if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+		qWarning("EDGE-DEBUG: node added to ctx %p: %p", (void *)this,
+				 (void *)node);
+	}
+
 	node_subs_[node].append(bridge_->subscribe(
 		reinterpret_cast<void *>(node),
 		OAKENGINE_EVENT_NODE_INPUT_CONNECTED));
@@ -436,6 +447,11 @@ void NodeViewContext::add_node_internal(OakEngineNode *node, NodeViewItem *item)
 				oak::Input ai(conn.node.handle(), conn.input_id, conn.element);
 				add_edge_internal(node, ai, item,
 								other_item->get_item_for_input(ai));
+			} else if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+				qWarning("EDGE-DEBUG: out-edge skipped (no item for %p): %p -> %p (%s) ctx=%p",
+						 (void *)conn.node.handle(), (void *)node,
+						 (void *)conn.node.handle(),
+						 qPrintable(conn.input_id), (void *)this);
 			}
 		}
 	}
@@ -449,6 +465,11 @@ void NodeViewContext::add_node_internal(OakEngineNode *node, NodeViewItem *item)
 				oak::Input ai(conn.node.handle(), conn.input_id, conn.element);
 				add_edge_internal(conn.source_node.handle(), ai, other_item,
 								item->get_item_for_input(ai));
+			} else if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+				qWarning("EDGE-DEBUG: in-edge skipped (no item for %p): %p -> %p (%s) ctx=%p",
+						 (void *)conn.source_node.handle(),
+						 (void *)conn.source_node.handle(), (void *)node,
+						 qPrintable(conn.input_id), (void *)this);
 			}
 		}
 	}
@@ -461,6 +482,12 @@ void NodeViewContext::add_edge_internal(OakEngineNode *output, const oak::Input
 		return;
 	}
 
+	if (qEnvironmentVariableIsSet("OAK_DEBUG_EDGES")) {
+		qWarning("EDGE-DEBUG: edge added: %p -> %p (%s,%d) ctx=%p", (void *)output,
+				 (void *)input.node_handle(), qPrintable(input.input_id()),
+				 input.element(), (void *)this);
+	}
+
 	NodeViewEdge *edge_ui = new NodeViewEdge(oak::Node(output), input, from, to, this);
 
 	edge_ui->adjust();

app/widget/viewer/viewer.cpp

@@ -110,7 +110,7 @@ static SampleBuffer preview_req_to_sample_buffer(OakEnginePreviewRequest *req)
 			oakcore_audioparams_free(ap);
 
 			// Determine sample count from first channel
-			int n = oakengine_preview_request_get_audio_samples(req, 0, nullptr, 1 << 30);
+			int n = oakengine_preview_request_get_audio_sample_count(req);
 			if (n > 0) {
 				oakcore_samplebuffer_set_sample_count(sb, size_t(n));
 				oakcore_samplebuffer_allocate(sb);