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/block/clip/clip.cpp

@@ -435,30 +435,36 @@ void ClipBlock::invalidate_cache(const TimeRange &range, const QString &from,
 			viewers.isEmpty() ? nullptr : viewers.first();
 
 		if (new_connected_viewer != connected_viewer_) {
+ 			TimelineMarkerList *old_markers =
+				connected_viewer_ ? connected_viewer_->get_markers() : nullptr;
+			if (old_markers) {
+				disconnect(old_markers, &TimelineMarkerList::marker_added, this,
+						   &ClipBlock::preview_changed);
+				disconnect(old_markers, &TimelineMarkerList::marker_removed,
+						   this, &ClipBlock::preview_changed);
+				disconnect(old_markers, &TimelineMarkerList::marker_modified,
+						   this, &ClipBlock::preview_changed);
+			}
 			if (connected_viewer_) {
-				disconnect(connected_viewer_->get_markers(),
-						   &TimelineMarkerList::marker_added, this,
-						   &ClipBlock::preview_changed);
-				disconnect(connected_viewer_->get_markers(),
-						   &TimelineMarkerList::marker_removed, this,
-						   &ClipBlock::preview_changed);
-				disconnect(connected_viewer_->get_markers(),
-						   &TimelineMarkerList::marker_modified, this,
-						   &ClipBlock::preview_changed);
+				disconnect(connected_viewer_, &ViewerOutput::destroyed, this,
+						   nullptr);
 			}
 
 			connected_viewer_ = new_connected_viewer;
 
+			TimelineMarkerList *new_markers =
+				connected_viewer_ ? connected_viewer_->get_markers() : nullptr;
+			if (new_markers) {
+				connect(new_markers, &TimelineMarkerList::marker_added, this,
+						&ClipBlock::preview_changed);
+				connect(new_markers, &TimelineMarkerList::marker_removed, this,
+						&ClipBlock::preview_changed);
+				connect(new_markers, &TimelineMarkerList::marker_modified,
+						this, &ClipBlock::preview_changed);
+			}
 			if (connected_viewer_) {
-				connect(connected_viewer_->get_markers(),
-						&TimelineMarkerList::marker_added, this,
-						&ClipBlock::preview_changed);
-				connect(connected_viewer_->get_markers(),
-						&TimelineMarkerList::marker_removed, this,
-						&ClipBlock::preview_changed);
-				connect(connected_viewer_->get_markers(),
-						&TimelineMarkerList::marker_modified, this,
-						&ClipBlock::preview_changed);
+				connect(connected_viewer_, &ViewerOutput::destroyed, this,
+						[this]() { connected_viewer_ = nullptr; });
 			}
 		}