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/render/previewautocacher.cpp

@@ -855,6 +855,25 @@ void PreviewAutoCacher::set_project(Project *project)
 	project_ = project;
 
 	if (project_) {
+		// If the project dies while we're still using it (e.g. shutdown order:
+		// project freed before the RenderManager), drop all state that
+		// references its nodes/caches without touching them. Otherwise the
+		// next set_project(nullptr) would disconnect signals on dead caches.
+		connect(project_, &Project::destroyed, this, [this]() {
+			project_ = nullptr;
+			delayed_requeue_timer_.stop();
+			single_frame_render_ = nullptr;
+			video_immediate_passthroughs_.clear();
+			pending_video_jobs_.clear();
+			pending_audio_jobs_.clear();
+			video_cache_data_.clear();
+			audio_cache_data_.clear();
+			multicam_ = nullptr;
+			// The copier's own destroyed-guard nulls its original_; this just
+			// clears its copy maps without touching the dead project.
+			copier_->set_project(nullptr);
+		});
+
 		// Copy graph (this should always be a Project)
 		set_renders_paused(true);