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/error.h

@@ -29,6 +29,16 @@
  * failure. String getters return the required buffer size in bytes
  * (including the terminating NUL) as a non-negative value instead.
  */
+/**
+ * @brief Current ABI version stamped into every oaknode handle.
+ *
+ * Bump whenever a handle layout or the semantics of any exported
+ * function change incompatibly. Consumers should compare a handle's
+ * abi_version field against the value they were compiled with before
+ * dereferencing ctx.
+ */
+#define OAKNODE_ABI_VERSION 1
+
 #define OAKNODE_OK 0 /**< Success. */
 #define OAKNODE_E_INVALID (-1) /**< NULL handle or invalid argument. */
 #define OAKNODE_E_STATE (-2) /**< Call not valid in the current state. */