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).

tests/CMakeLists.txt

@@ -53,8 +53,13 @@ function(olive_add_test GROUP NAME SOURCE)
   target_link_libraries(
     ${NAME}
     PRIVATE
+    oakengine-obj
     ${OLIVE_LIBRARIES}
   )
+  # Export symbols so dlopen'd render backend plugins can resolve engine objects
+  if (UNIX AND NOT APPLE)
+    target_link_options(${NAME} PRIVATE "LINKER:--export-dynamic")
+  endif()
   target_compile_definitions(
     ${NAME}
     PRIVATE

tests/gtest/CMakeLists.txt

@@ -159,6 +159,7 @@ target_include_directories(
 target_link_libraries(
   olive-gtest
   PRIVATE
+  oakengine-obj
   ${OLIVE_LIBRARIES}
   GTest::gtest
   Qt${QT_VERSION_MAJOR}::Test
@@ -177,6 +178,11 @@ target_compile_options(
   ${OLIVE_COMPILE_OPTIONS}
 )
 
+# Export symbols so dlopen'd render backend plugins can resolve engine objects
+if (UNIX AND NOT APPLE)
+  target_link_options(olive-gtest PRIVATE "LINKER:--export-dynamic")
+endif()
+
 if (OAK_ENABLE_DYNAMIC_RENDER_BACKEND)
   target_compile_definitions(olive-gtest PRIVATE OAK_ENABLE_DYNAMIC_RENDER_BACKEND)
   add_dependencies(olive-gtest oakgl)