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/render/copier.h

@@ -45,17 +45,19 @@ OakRenderProjectCopier *oakrender_project_copier_create(void);
 /** @brief Free the copier AND its copied project. NULL-safe. */
 void oakrender_project_copier_free(OakRenderProjectCopier *copier);
 
-/** @brief (Re)build the copy from `project`. */
+/** @brief (Re)build the copy from `project` (borrowed handle). */
 int oakrender_project_copier_set_project(OakRenderProjectCopier *copier,
-										 OakNodeProject *project);
+										 OakNodeProject project);
 
-/** @brief The copied counterpart of an original node (borrowed), NULL
- *        when the node is not in the copied project. */
-OakNodeNode *oakrender_project_copier_get_copy(
-	OakRenderProjectCopier *copier, OakNodeNode *original);
+/** @brief The copied counterpart of an original node (borrowed handle;
+ *        freeing it only releases the handle box), empty handle when the
+ *        node is not in the copied project. */
+OakNodeNode oakrender_project_copier_get_copy(
+	OakRenderProjectCopier *copier, OakNodeNode original);
 
-/** @brief The copied project (borrowed). */
-OakNodeProject *oakrender_project_copier_get_copied_project(
+/** @brief The copied project (borrowed handle; freeing it only releases
+ *        the handle box). */
+OakNodeProject oakrender_project_copier_get_copied_project(
 	OakRenderProjectCopier *copier);
 
 #ifdef __cplusplus