refactor(node,render): route oaknode->oakrender calls through the C ABI

- oakrender cache C API: add create_for_node (four cache kinds),
  get_uuid, request, load/save_state, set_saving_enabled,
  set_passthrough, get_passthroughs, get_valid_cache_filename,
  get_timebase, lock/unlock, invalidate_range (rational variant),
  get_native (C++ only, for the in-module PreviewAutoCacher)
- Node's four caches become owned OakRenderCache value handles; all
  cache access in node.cpp/clip/viewer/traverser/serializers goes
  through the C ABI
- oaknode_node_get_video_frame_cache returns an addref'd
  OakRenderCache; drop OakNodeFrameCache; oaktask precache/export and
  oakrender_video_ticket_params take the value handle
- color: OCIOBaseNode/OCIOLutNode hold OakColorProcessor handles
  (dtors added); oakrender gains color_processor_create_transform/
  create_lut/create_grading_primary/get_native and LUT library
  queries, keeping all OCIO transform construction inside oakrender;
  oaknode gains colormanager_wrap_borrowed/get_native
- RenderManager::instance() check -> oakrender_manager_available();
  cancel_video_tasks and disk cache path go through manager C API
- remaining node->render C++ symbol: only olive::Texture::~Texture
  via Variant's shared_ptr payload (recorded exception, same class as
  the UndoCommand inheritance)

Tests: new cache/color/manager/colormanager cases; all standalone
trees green (common 196, codec 22, audio 40, task 112, render 63,
timeline 125, node 98, plugin 102).

include/node/colormanager.h

@@ -72,6 +72,16 @@ OakNodeColorManager oaknode_colormanager_init(OakNodeProject project);
  */
 void oaknode_colormanager_free(OakNodeColorManager *manager);
 
+/**
+ * @brief Borrowed handle wrapping a native manager pointer held by a
+ *        node (olive::OCIOBaseNode::manager()).
+ *
+ * The manager stays owned by its project: release() on this handle
+ * only frees the box. Empty handle (ctx == NULL) for a NULL native
+ * pointer.
+ */
+OakNodeColorManager oaknode_colormanager_wrap_borrowed(void *native_manager);
+
 /**
  * @brief Load the built-in default OCIO config and set the default input
  * colorspace (olive::ColorManager::init()).
@@ -189,6 +199,21 @@ int oaknode_colormanager_get_compliant_color_transform(
 	OakNodeColorManager manager, OakColorTransform transform,
 	int force_display, OakColorTransform *out);
 
+#ifdef __cplusplus
+} /* extern "C" */
+
+namespace olive { class ColorManager; }
+
+extern "C" {
+#endif
+
+/**
+ * @brief Borrowed access to the underlying C++ manager (C++ only, for
+ *        adapter layers). Valid while the handle is held. NULL-safe.
+ */
+olive::ColorManager *oaknode_colormanager_get_native(
+	OakNodeColorManager manager);
+
 #ifdef __cplusplus
 }
 #endif