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

src/render/transition/common/cancelableobject.h

@@ -0,0 +1,33 @@
+#pragma once
+// Transitional: engine/common/cancelableobject.h de-Qt target form. Header-only
+// in the original; the only Qt dependency was the transitively included
+// render/cancelatom.h, which oakrender provides for real. M-common folds this
+// into oakcommon; until then this contract matches the original 1:1.
+#include "cancelatom.h"
+
+namespace olive {
+
+class CancelableObject {
+public:
+	CancelableObject() {}
+
+	virtual ~CancelableObject() = default;
+
+	void cancel()
+	{
+		cancel_.cancel();
+		CancelEvent();
+	}
+
+	CancelAtom *get_cancel_atom() { return &cancel_; }
+
+	bool is_cancelled() { return cancel_.is_cancelled(); }
+
+protected:
+	virtual void CancelEvent() {}
+
+private:
+	CancelAtom cancel_;
+};
+
+}