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/tests/oakengine_node_test.cpp

@@ -280,6 +280,22 @@ static void test_edges(OakEngineProject *project, OakEngineNode *solid,
 	EXPECT_TRUE(oakengine_node_connect(solid, lut, "tex_in") == OAKENGINE_OK);
 	EXPECT_TRUE(oakengine_node_input_is_connected(lut, "tex_in") == 1);
 
+	// Output-side enumeration must report the DESTINATION node (the
+	// connection's input side), never the source itself — the node view
+	// builds half its edges through this path.
+	EXPECT_TRUE(oakengine_node_output_connection_count(solid) > 0);
+	{
+		OakEngineNode *dst = NULL;
+		char id_buf[64];
+		int element = -99, hidden = -1;
+		EXPECT_TRUE(oakengine_node_output_connection_at_ex(
+					solid, 0, &dst, id_buf, sizeof(id_buf), &element,
+					&hidden) == OAKENGINE_OK);
+		EXPECT_TRUE(dst == lut);
+		EXPECT_TRUE(strcmp(id_buf, "tex_in") == 0);
+		EXPECT_TRUE(element == -1);
+	}
+
 	// Already-connected input is refused; unknown ids and unconnectable
 	// inputs fail.
 	EXPECT_TRUE(oakengine_node_connect(solid, lut, "tex_in") ==