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/manageddisplay/manageddisplay.h

@@ -33,10 +33,10 @@
 #endif
 
 #include "oakengine/color.h"
+#include "oakengine/display.h"
 #include "oakengine/events.h"
 #include "render/colorprocessor.h"
 #include "render/colortransform.h"
-#include "render/renderer.h"
 #include "widget/manageddisplay/colorprocessorhandle.h"
 #include "widget/menu/menu.h"
 
@@ -222,7 +222,7 @@ protected:
    */
 	virtual void color_processor_changed_event();
 
-	Renderer *renderer() const
+	void *renderer() const
 	{
 		return attached_renderer_;
 	}
@@ -261,7 +261,7 @@ protected:
 		return wrapper_ ? wrapper_->rect() : QRect();
 	}
 
-	VideoParams get_viewport_params() const;
+	oak_video_params get_viewport_params() const;
 
 protected slots:
 	/**
@@ -303,7 +303,7 @@ private:
 	/**
    * @brief Renderer abstraction
    */
-	Renderer *attached_renderer_;
+	void *attached_renderer_;
 
 	/**
    * @brief Connected color manager