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

src/timeline/c_api/workarea.cpp

@@ -54,9 +54,9 @@ olive::core::Rational rat(int64_t n, int64_t d)
 
 } // namespace
 
-OakTimelineWorkArea *oaktimeline_workarea_of(OakNodeNode *owner)
+OakTimelineWorkArea *oaktimeline_workarea_of(OakNodeNode owner)
 {
-	if (!owner) {
+	if (!owner.ctx) {
 		return NULL;
 	}