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).

engine/oakengine.ver

@@ -1,6 +1,15 @@
 {
     global:
         oakengine_*;
+        /* Render backend plugin ABI (oakgl / oakvulkan dlopen) */
+        _ZN5olive8Renderer*;
+        _ZTVN5olive8RendererE;
+        _ZN5olive11VideoParams19get_bytes_per_pixelE*;
+        _ZN5olive11VideoParams21get_bytes_per_channelE*;
+        _ZN5olive13FileFunctions19read_file_as_stringE*;
+        _ZN5olive7Texture23k_default_interpolationE;
+        /* App residual C++ dependency (to be migrated to C ABI) */
+        _ZNK5olive6Folder17has_child_recursiveE*;
     local:
         *;
 };