engine: add the export family to the C ABI facade; mp4 transcode

- oakengine_export_render drives ExportTask synchronously (offline
  render + encode) with a progress callback, codec probing, and a
  thread-local error channel; exporter.h keeps clear of the visibility
  macro header
- oak-cli transcode now defaults to mp4 (H.264/AAC) with --format ppm
  keeping the raw output path
- two real concurrency bugs found by the facade's own test: ExportTask
  deadlocks when start()ed synchronously (queued conform handshake
  needs an event loop), and the progress callback must be captured by
  value because it fires on the task thread

engine/CMakeLists.txt

@@ -274,4 +274,31 @@ if (BUILD_TESTS)
     target_compile_definitions(oakengine_timeline_edit_test PRIVATE
             OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
     )
+
+    make_oakengine_test(oakengine_export_test)
+    # The export test builds sequence content through the engine C++ API and
+    # probes the dynamic render backend like oakengine_renderer_test does.
+    target_include_directories(oakengine_export_test PRIVATE
+            ${CMAKE_SOURCE_DIR}/third_party/openfx/include
+            ${CMAKE_SOURCE_DIR}/third_party/openfx/HostSupport/include
+            ${OLIVE_INCLUDE_DIRS}
+    )
+    target_compile_definitions(oakengine_export_test PRIVATE
+            ${OLIVE_DEFINITIONS}
+            OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
+    )
+    target_compile_options(oakengine_export_test PRIVATE
+            ${OLIVE_COMPILE_OPTIONS}
+    )
+    if (OAK_ENABLE_DYNAMIC_RENDER_BACKEND)
+        target_compile_definitions(oakengine_export_test PRIVATE
+                OAK_ENABLE_DYNAMIC_RENDER_BACKEND)
+        add_dependencies(oakengine_export_test oakgl)
+        if (TARGET oakvulkan)
+            add_dependencies(oakengine_export_test oakvulkan)
+        endif ()
+    endif ()
+    if (TARGET olive-render-worker)
+        add_dependencies(oakengine_export_test olive-render-worker)
+    endif ()
 endif ()