refactor(node): switch oaknode to refcounted value handles, migrate consumers

- all 15 OakNode* handle types become neutral by-value structs
  {ctx, addref, release, abi_version}; shared box in
  src/node/c_api/nodehandle.h with owns flag (borrowed accessors
  return non-owning boxes; graph insertion flips owns off)
- oaktimeline/oaktask/oakrender call sites and their own public
  headers migrated to value handles; identity comparisons in
  timeline/task now compare native pointers
- regressions green: oaknode 96, oaktimeline 117, oaktask 106,
  oakrender 44, oakcommon 193, oakcodec 18, oakaudio 36

include/timeline/workarea.h

@@ -37,10 +37,10 @@ extern "C" {
 typedef struct OakTimelineWorkArea OakTimelineWorkArea;
 
 /**
- * @brief Borrowed work area of a viewer node (sequence). NULL for NULL
- * or when the node is not a viewer.
+ * @brief Borrowed work area of a viewer node (sequence). NULL for an
+ * empty handle or when the node is not a viewer.
  */
-OakTimelineWorkArea *oaktimeline_workarea_of(OakNodeNode *owner);
+OakTimelineWorkArea *oaktimeline_workarea_of(OakNodeNode owner);
 
 /**
  * @brief Read the work area state. Out params may individually be NULL.