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

engine/include/oakengine/preview.h

@@ -78,6 +78,8 @@ typedef struct oak_playback_frame {
 	int format;       /**< olive::PixelFormat::Format value. */
 	const void *data; /**< Planar data pointer (first plane). */
 	int linesize;     /**< Bytes per row of the first plane. */
+	int64_t timestamp_num; /**< Frame timestamp (seconds) as a rational. */
+	int64_t timestamp_den; /**< 0 when the frame carries no timestamp. */
 } oak_playback_frame;
 
 /**