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/node/factory.h

@@ -34,9 +34,11 @@ extern "C" {
  * internal node-type library.
  *
  * The library must be populated with oaknode_factory_initialize() before
- * any other call; oaknode_factory_destroy() releases it. Prototype nodes
- * from oaknode_factory_node_at() are owned by the library: read-only
- * metadata queries only, never free them or add them to a graph.
+ * any other call; oaknode_factory_destroy() releases it. The factory is
+ * a process-wide singleton (static olive::NodeFactory), so there is no
+ * OakNodeFactory handle type. Prototype nodes from
+ * oaknode_factory_node_at() are owned by the library: read-only metadata
+ * queries only, never add them to a graph.
  */
 
 /**
@@ -77,17 +79,19 @@ int oaknode_factory_name_from_id(const char *type_id, char *buf,
 /**
  * @brief Create a node of `type_id` WITHOUT adding it to any graph
  * (NodeFactory::create_from_id()). The caller owns the returned node
- * and must release it with oaknode_node_free() while it is still
- * orphaned. Returns NULL when the id is unknown or not initialized.
+ * (reference count 1) and must release it with oaknode_node_free() while
+ * it is still orphaned. ctx is NULL when the id is unknown or not
+ * initialized.
  */
-OakNodeNode *oaknode_factory_create_from_id(const char *type_id);
+OakNodeNode oaknode_factory_create_from_id(const char *type_id);
 
 /**
- * @brief Borrow the prototype node at `index` in the library.
+ * @brief Borrow the prototype node at `index` in the library (non-owning
+ * handle written to `out_node`; release it with oaknode_node_free()).
  * OAKNODE_E_NOT_FOUND for an out-of-range index, OAKNODE_E_STATE when
  * not initialized.
  */
-int oaknode_factory_node_at(int index, OakNodeNode **out_node);
+int oaknode_factory_node_at(int index, OakNodeNode *out_node);
 
 #ifdef __cplusplus
 }