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/render/renderprocessor.cpp

@@ -37,6 +37,7 @@
 #include "pluginSupport/oliveclip.h"
 #include "pluginSupport/olivehost.h"
 #include "oliveimpl/render/ipc/frameslotpool.h"
+#include "src/capi/displayinternal.h"
 
 namespace olive
 {
@@ -367,7 +368,7 @@ NodeValueDatabase RenderProcessor::generate_database(const Node *node,
 		if (QtUtils::value_to_ptr<MultiCamNode>(ticket_->property("multicam")) ==
 			multicam) {
 			int sz = multicam->get_source_count();
-			QVector<TexturePtr> multicam_tex(sz);
+			QVector<void *> multicam_tex(sz);
 			for (int i = 0; i < sz; i++) {
 				NodeValueTable t =
 					generate_table(multicam->get_connected_render_output(
@@ -377,7 +378,9 @@ NodeValueDatabase RenderProcessor::generate_database(const Node *node,
 					multicam, multicam->k_sources_input, i, &t, range);
 				resolve_jobs(val);
 
-				multicam_tex[i] = val.to_texture();
+				TexturePtr tp = val.to_texture();
+				// Store as opaque retained handle for the C ABI app layer
+				multicam_tex[i] = oakengine_internal_wrap_texture(tp);
 			}
 			ticket_->setProperty("multicam_output",
 								 QVariant::fromValue(multicam_tex));