refactor(render): de-Qt oakrender and wrap it in a pure C ABI

- copy engine/render to src/render/src (sunk param types excluded),
  de-Qt in five parallel groups: core machinery (tickets/worker pool/
  jobs), caches, color/texture, preview/IPC, GPU backends
- replace Qt GL/Vulkan wrappers with native context abstractions
  (CGL/EGL/WGL, raw vulkan.h), QProcess with POSIX WorkerProcess,
  QJsonObject with a minimal NDJSON-compatible workerjson (wire
  protocol unchanged), QDataStream disk state with a byte-compatible
  BinaryStream
- signals become single std::function callbacks or facade-triggered
  calls per the documented signal/slot strategy
- pure C ABI in include/render + src/render/c_api (renderer/cache/
  color/manager families, OAKRENDER_E_* codes), 37 gtest cases
- bridge src/node/transition/render/* stubs to the real oakrender
  headers, closing the node<->render cycle: liboaknode links
  liboakrender, zero dangling symbols
- docs: M7 implementation status + oakrender semantic-change notes

include/render/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_RENDER_ERROR_H
+#define OAK_EDITOR_RENDER_ERROR_H
+
+/**
+ * @brief Status and error codes shared by all oakrender C API families.
+ *
+ * Return-code convention (mirrors include/node/error.h):
+ * 0 (OAKRENDER_OK) on success, a negative OAKRENDER_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 OAKRENDER_OK 0 /**< Success. */
+#define OAKRENDER_E_INVALID (-1) /**< NULL handle or invalid argument. */
+#define OAKRENDER_E_STATE (-2) /**< Call not valid in the current state. */
+#define OAKRENDER_E_FAILED (-3) /**< The underlying operation failed. */
+#define OAKRENDER_E_NOT_FOUND (-4) /**< Index out of range / entry not found. */
+#define OAKRENDER_E_NOMEM (-5) /**< Allocation failed. */
+
+#endif //OAK_EDITOR_RENDER_ERROR_H