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/c_api/CMakeLists.txt

@@ -0,0 +1,6 @@
+target_sources(oakrender PRIVATE
+        renderer.cpp
+        cache.cpp
+        color.cpp
+        manager.cpp
+)