refactor(node): de-Qt oaknode and wrap it in a pure C ABI

- copy engine/node (188 files) to src/node/src, de-Qt in waves:
  core infra (Node/Param/Value/Variant/mathtypes), project/serializer,
  block/output, color, effect leaves, generator, gizmo/plugins
- strip QObject/signals/slots: notifications move to the facade's
  oakengine_event channel, ownership becomes explicit (unique_ptr,
  add_keyframe/add_gizmo), sender() replaced by current_gizmo
- QVariant replaced by olive::Variant, Qt math types by POD mathtypes,
  QXmlStreamReader/Writer by oakcommon's expat-based classes
- sink VideoParams/SubtitleParams/LoopMode/ColorTransform to oakcommon
  (M3.5); polygon/text rasterization behind backend hooks
- pure C ABI in include/node + src/node/c_api (oaknode_ prefix,
  OAKNODE_E_* codes, undoable variants take OakUndoCommand out-params)
- fix Project::clear() root_ reset + disconnect assert, Sequence
  TrackList leak
- 96 gtest cases green in standalone build (build-oaknode)
- docs: signal/slot handling strategy + M3 implementation status

include/node/error.h

@@ -0,0 +1,39 @@
+/***
+
+  Oak Video Editor - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_EDITOR_NODE_ERROR_H
+#define OAK_EDITOR_NODE_ERROR_H
+
+/**
+ * @brief Status and error codes shared by all oaknode C API families.
+ *
+ * Return-code convention (mirrors engine/include/oakengine/init.h):
+ * 0 (OAKNODE_OK) on success, a negative OAKNODE_E_* error code on
+ * failure. String getters return the required buffer size in bytes
+ * (including the terminating NUL) as a non-negative value instead.
+ */
+#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. */
+#define OAKNODE_E_FAILED (-3) /**< The underlying operation failed. */
+#define OAKNODE_E_NOT_FOUND (-4) /**< Index out of range / entry not found. */
+#define OAKNODE_E_NOMEM (-5) /**< Allocation failed. */
+
+#endif //OAK_EDITOR_NODE_ERROR_H