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/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();