R7: pure C ABI display boundary + engine visibility closure

R7-A (Qwen 3.8 Max): oakengine/display.h rewritten to the POD contract
from r7-pure-abi-plan.md - oak_video_params everywhere, opaque
texture/frame handles with retain/free protocol (engine-heap control
blocks), OakSharedBuffer refcounted wrapper for the QVariant playback
path. All TexturePtr/FramePtr gone from app (47 sites).

R7-B (Qwen 3.8 Max): liboakengine.so exports 3486 -> 19 C++ symbols
(version script oakengine.ver: oakengine_* plus the documented
oakgl/oakvulkan dlopen plugin ABI). oakengine-obj OBJECT library feeds
both the shared lib and the test binaries (-rdynamic so dlopen'd
backends resolve engine objects).

Fix (Kimi K3): producer/consumer type mismatch - viewerdisplay
unpacks OakSharedBufferPtr but viewer.cpp pushed raw void* handles,
so no frame ever reached the display widget (all 5 vulkan viewer
tests timed out with 'never received a texture'). Producers now wrap
with oak_make_shared_frame / oak_make_shared_texture(retain).

Verified: build 0 errors, ctest 45/45, nm U _ZN5olive = 0 in
oak-editor/oak-render-worker/oak-cli, 19 exported C++ symbols in
liboakengine.so (all documented plugin ABI).

app/widget/viewer/viewerqueue.h

@@ -26,13 +26,13 @@
 #include <QMutex>
 #include <QMutexLocker>
 
-#include "codec/frame.h"
+#include <olive/core/core.h>
 
 namespace olive
 {
 
 struct ViewerPlaybackFrame {
-	Rational timestamp;
+	core::Rational timestamp;
 	QVariant frame;
 };
 
@@ -73,7 +73,7 @@ public:
 		}
 	}
 
-	void purge_before(const Rational &time, int playback_speed)
+	void purge_before(const core::Rational &time, int playback_speed)
 	{
 		QMutexLocker locker(mutex_);
 		while (!this->empty() &&