engine: begin the liboakengine C ABI facade with the IPC subsystem

- oakengine/export.h establishes the OAKENGINE_API visibility macros;
  include/oakengine/ipc.h is the first pure-C surface (41 functions:
  shm, frame slot pool, and the worker IPC messages as POD<->JSON
  build/parse), implemented in engine/src/capi/
- the IPC implementations move to engine/src/oliveimpl (namespace
  olive::engine::internal::ipc); engine/render/ipc/*.h are rebuilt as
  same-name/same-API wrapper classes forwarding across the C boundary
- FrameSlotMeta is shared with the C header verbatim so the app/worker
  wire format (v1) is bit-identical; static_asserts pin sizeof and
  field offsets
- spscringbuffer.h moves to include/oakengine/ as an inline-only
  header (no symbols, not ABI)
- new pure-C test oakengine_ipc_test (make_oakengine_test, no GL)
  covers shm, frame pool, message round-trips and the layout asserts;
  full gtest suite stays green (1986 tests)

tests/gtest/render_ipc_test.cpp

@@ -22,9 +22,9 @@
 #include <QJsonDocument>
 #include <QJsonObject>
 
+#include "oakengine/spscringbuffer.h"
 #include "render/ipc/frameslotpool.h"
 #include "render/ipc/ipcmessage.h"
-#include "render/ipc/spscringbuffer.h"
 
 using namespace olive::ipc;