fix: memory-safety and playback regressions found via ASan

- nodeparamview: add visited set to get_distance_between_nodes, fixing
  unbounded recursion (stack overflow) when the node graph has a cycle
- viewerdisplay: give texture_ a consistent owner via assign_texture();
  borrowed queue textures are now retained, created ones freed, fixing
  a dangling pointer that corrupted the heap and crashed in the GL driver
- playbackcache: resignal_requests() iterates a copy, handlers may
  clear_request_range() while iterating (ASan container-overflow)
- preview C API: preview request ticket lambdas captured the request
  state raw; after oakengine_preview_request_free the ticket outlived
  the request and the finished callback wrote into freed memory
  (heap-use-after-free). The finished flag is now a shared_ptr captured
  weakly by the callbacks
- playback: oak_playback_frame regains a timestamp (num/den) filled
  from olive::Frame; the viewer queue append no longer uses Rational()
  for every frame, which made append_timewise drop all but the first
  frame and froze the picture during playback
- mainwindow: open_node_in_viewer refuses sequence nodes; sequences
  already have the Sequence Viewer, and saved layouts could otherwise
  resurrect a redundant floating Viewer bound to the sequence

app/widget/nodeparamview/nodeparamview.cpp

@@ -951,18 +951,25 @@ void NodeParamView::add_node(OakEngineNode *n, OakEngineNode *ctx, NodeParamView
 	}
 }
 
-int get_distance_between_nodes(OakEngineNode *start, OakEngineNode *end)
+int get_distance_between_nodes(OakEngineNode *start, OakEngineNode *end,
+							   QSet<OakEngineNode *> *visited)
 {
 	if (start == end) {
 		return 0;
 	}
 
+	if (visited->contains(start)) {
+		return -1;
+	}
+	visited->insert(start);
+
 	const oak::Node start_node(start);
 	const int connection_count = start_node.input_connection_count_all();
 	for (int i = 0; i < connection_count; i++) {
 		OakEngineNode *upstream =
 			start_node.input_connection_at_all(i).node.handle();
-		int this_node_dist = get_distance_between_nodes(upstream, end);
+		int this_node_dist =
+			get_distance_between_nodes(upstream, end, visited);
 		if (this_node_dist != -1) {
 			return 1 + this_node_dist;
 		}
@@ -971,6 +978,12 @@ int get_distance_between_nodes(OakEngineNode *start, OakEngineNode *end)
 	return -1;
 }
 
+int get_distance_between_nodes(OakEngineNode *start, OakEngineNode *end)
+{
+	QSet<OakEngineNode *> visited;
+	return get_distance_between_nodes(start, end, &visited);
+}
+
 void NodeParamView::sort_items_in_context(NodeParamViewContext *context_item)
 {
 	QVector<QPair<NodeParamViewItem *, int>> distances;

app/widget/viewer/viewer.cpp

@@ -1836,7 +1836,9 @@ void ViewerWidget::renderer_generated_frame_for_queue()
 				QVariant frame = QVariant::fromValue(f);
 
 				foreach (ViewerDisplayWidget *dw, playback_devices_) {
-					dw->queue()->append_timewise({ Rational(), frame },
+					dw->queue()->append_timewise({ Rational(pf.timestamp_num,
+															pf.timestamp_den),
+												   frame },
 												playback_speed_);
 				}
 			}

app/widget/viewer/viewerdisplay.cpp

@@ -108,6 +108,18 @@ ViewerDisplayWidget::~ViewerDisplayWidget()
 	MANAGEDDISPLAYWIDGET_DEFAULT_DESTRUCTOR_INNER;
 }
 
+void ViewerDisplayWidget::assign_texture(void *t, bool owned)
+{
+	if (owned_texture_) {
+		oakengine_display_texture_free(owned_texture_);
+	}
+	if (t && !owned) {
+		oakengine_display_texture_retain(t);
+	}
+	owned_texture_ = t;
+	texture_ = t;
+}
+
 void ViewerDisplayWidget::set_matrix_translate(const QMatrix4x4 &mat)
 {
 	translate_matrix_ = mat;
@@ -483,10 +495,11 @@ void ViewerDisplayWidget::on_paint()
 					 oakengine_display_texture_height(texture_) != frame_vp.height ||
 					 oakengine_display_texture_format(texture_) != frame_vp.format ||
 					 oakengine_display_texture_channel_count(texture_) != 4)) {
-					texture_ = oakengine_display_texture_create(
-						renderer(), &frame_vp,
-						oakengine_codec_frame_data(load_handle),
-						oakengine_codec_frame_linesize(load_handle));
+					assign_texture(oakengine_display_texture_create(
+									   renderer(), &frame_vp,
+									   oakengine_codec_frame_data(load_handle),
+									   oakengine_codec_frame_linesize(load_handle)),
+								   true);
 				} else if (!drew_backend_neutral_frame) {
 					oakengine_display_texture_upload(
 						texture_, oakengine_codec_frame_data(load_handle),
@@ -499,7 +512,7 @@ void ViewerDisplayWidget::on_paint()
 					src_ren && src_ren != renderer()) {
 					if (oakengine_display_renderer_is_open_gl(src_ren) &&
 						oakengine_display_renderer_is_open_gl(renderer())) {
-						texture_ = load_handle;
+						assign_texture(load_handle, false);
 					} else {
 						// Cross-backend: download and re-upload
 						void *tmp_frame = oakengine_codec_frame_create();
@@ -513,24 +526,25 @@ void ViewerDisplayWidget::on_paint()
 								&tex_params,
 								oakengine_codec_frame_data(tmp_frame),
 								oakengine_codec_frame_linesize(tmp_frame));
-							texture_ = oakengine_display_texture_create(
-								renderer(), &tex_params,
-								oakengine_codec_frame_data(tmp_frame),
-								oakengine_codec_frame_linesize(tmp_frame));
+							assign_texture(oakengine_display_texture_create(
+											   renderer(), &tex_params,
+											   oakengine_codec_frame_data(tmp_frame),
+											   oakengine_codec_frame_linesize(tmp_frame)),
+										   true);
 						} else {
-							texture_ = load_handle;
+							assign_texture(load_handle, false);
 						}
 						oakengine_codec_frame_free(tmp_frame);
 					}
 				} else if (!drew_backend_neutral_frame) {
-					texture_ = load_handle;
+					assign_texture(load_handle, false);
 				}
 			} else {
-				texture_ = load_custom_texture_from_frame(load_frame_);
+				assign_texture(load_custom_texture_from_frame(load_frame_), true);
 			}
 
 			if (drew_backend_neutral_frame) {
-				texture_ = nullptr;
+				assign_texture(nullptr, true);
 			}
 
 			emit texture_changed(texture_);
@@ -762,7 +776,7 @@ void ViewerDisplayWidget::on_destroy()
 
 	super::on_destroy();
 
-	texture_ = nullptr;
+	assign_texture(nullptr, true);
 	if (deinterlace_texture_) {
 		oakengine_display_texture_free(deinterlace_texture_);
 		deinterlace_texture_ = nullptr;

app/widget/viewer/viewerdisplay.h

@@ -338,9 +338,29 @@ private:
 
 	/**
    * @brief Internal reference to the OpenGL texture to draw. Set in SetTexture() and used in paintGL().
+   *
+   * Never assign directly; use assign_texture() so ownership stays consistent.
    */
 	void *texture_;
 
+	/**
+   * @brief Owned (retained or created) reference backing texture_.
+   *
+   * texture_ may point to a texture owned by the playback queue's
+   * OakSharedBuffer; this member holds our own reference so texture_ can
+   * never dangle after the queue entry is replaced.
+   */
+	void *owned_texture_ = nullptr;
+
+	/**
+   * @brief Assign texture_ with consistent ownership.
+   *
+   * Releases the previous owned reference. If owned is true, takes over the
+   * passed reference (e.g. freshly created); otherwise retains it. texture_
+   * is set to the same pointer (possibly nullptr).
+   */
+	void assign_texture(void *t, bool owned);
+
 	/**
    * @brief Internal texture to deinterlace to
    */