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/src/capi/node.cpp

@@ -5172,9 +5172,10 @@ int oakengine_node_output_connection_at(
 		return OAKENGINE_E_NOT_FOUND;
 	}
 	const auto &conn = conns[size_t(index)];
-	// conn.first = input Node*, conn.second = NodeInput on that node
+	// output_connections() stores {source (== self), NodeInput}; the
+	// connection's destination is the NodeInput's node, NOT conn.first.
 	if (input_node) {
-		*input_node = wrap(conn.first);
+		*input_node = wrap(conn.second.node());
 	}
 	if (input_id_buf && input_id_size > 0) {
 		string_to_buf(conn.second.input(), input_id_buf, input_id_size);
@@ -5197,9 +5198,10 @@ int oakengine_node_output_connection_at_ex(
 		return OAKENGINE_E_NOT_FOUND;
 	}
 	const auto &conn = conns[size_t(index)];
-	// conn.first = input Node*, conn.second = NodeInput on that node
+	// output_connections() stores {source (== self), NodeInput}; the
+	// connection's destination is the NodeInput's node, NOT conn.first.
 	if (input_node) {
-		*input_node = wrap(conn.first);
+		*input_node = wrap(conn.second.node());
 	}
 	if (input_id_buf && input_id_size > 0) {
 		string_to_buf(conn.second.input(), input_id_buf, input_id_size);

engine/src/capi/preview.cpp

@@ -619,6 +619,25 @@ int oakengine_preview_request_get_audio_channel_count(
 	return s->samples.is_allocated() ? s->samples.channel_count() : 0;
 }
 
+int oakengine_preview_request_get_audio_sample_count(
+	const OakEnginePreviewRequest *req)
+{
+	if (!req) {
+		return 0;
+	}
+	const OakEnginePreviewRequestState *s =
+		reinterpret_cast<const OakEnginePreviewRequestState *>(req);
+	if (!s->ticket || !s->ticket->has_result() || !s->finished->load()) {
+		return 0;
+	}
+	if (!s->has_audio) {
+		const_cast<OakEnginePreviewRequestState *>(s)->samples =
+			s->ticket->get().value<olive::SampleBuffer>();
+		const_cast<OakEnginePreviewRequestState *>(s)->has_audio = true;
+	}
+	return s->samples.is_allocated() ? int(s->samples.sample_count()) : 0;
+}
+
 int oakengine_preview_request_get_audio_sample_rate(
 	const OakEnginePreviewRequest *req)
 {