core: turn olivecore into liboakcore.so with a pure C ABI

liboakcore is now a shared library that exposes only a C ABI:
- every value class (Rational, TimeRange, Color, Bezier, AudioParams,
  SampleBuffer) and the free-function groups (StringUtils, fraction
  utils, Timecode) is wrapped in an opaque-handle C API under
  core/include/olive/core/oakcore/ (init/copy/free + self-first
  functions), implemented in core/src/capi/
- consumers keep the original C++ API unchanged through same-name
  wrapper classes that hold the handle and forward across the C
  boundary; original implementations moved to core/src/oliveimpl
  (namespace olive::core::internal) and are hidden from export
- TimeRangeList/TimeRangeListFrameIterator are reimplemented inline
  over the wrapper (iterators/containers don't cross C ABI)
- generic Value container stays internal (unused by consumers) and is
  no longer part of the public umbrella header
- hidden visibility + OAKCORE_BUILD export macro; nm shows zero
  olive::* symbols exported
- install into the platform's standard libdir (GNUInstallDirs);
  Windows DLLs next to the executables, macOS into the app bundle
- TimelineWorkArea::in/out/length now return by value: the wrapped
  TimeRange getters return values, and forwarding them through const
  references dangled (found via RenderWorkerFootageTest crash)
- tests: 9 new pure C ABI test executables (oakcore_*_test) covering
  every public C function; 4 stale legacy core tests removed (they
  targeted a long-renamed API and were never built due to a malformed
  option() that also kept OLIVECORE_BUILD_TESTS off)
- CI/CD: oakcore.dll staged for NSIS, liboakcore.so added to the
  AppImage validation list, build-tree DLL copies on Windows

app/CMakeLists.txt

@@ -295,7 +295,8 @@ elseif (APPLE)
             COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:olive-render-worker> $<TARGET_BUNDLE_DIR:olive-editor>/Contents/MacOS/
             COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:oakgl> $<TARGET_BUNDLE_DIR:olive-editor>/Contents/MacOS/
             COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:ffmpeg_bridge> $<TARGET_BUNDLE_DIR:olive-editor>/Contents/MacOS/
-            COMMENT "Copying oak-render-worker, render backends, and ffmpeg_bridge into Oak.app"
+            COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:olivecore> $<TARGET_BUNDLE_DIR:olive-editor>/Contents/MacOS/
+            COMMENT "Copying oak-render-worker, render backends, ffmpeg_bridge and liboakcore into Oak.app"
     )
     if (TARGET oakvulkan)
         add_custom_command(TARGET olive-editor POST_BUILD
@@ -307,6 +308,14 @@ elseif (UNIX)
     install(TARGETS olive-editor RUNTIME DESTINATION bin)
 endif ()
 
+if (WIN32)
+    # Windows has no RPATH: shared libraries must sit next to the executable
+    add_custom_command(TARGET olive-editor POST_BUILD
+            COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:ffmpeg_bridge> $<TARGET_FILE_DIR:olive-editor>
+            COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:olivecore> $<TARGET_FILE_DIR:olive-editor>
+    )
+endif ()
+
 # Set link libraries
 target_link_libraries(olive-editor PRIVATE ${OLIVE_LIBRARIES})
 target_link_libraries(libolive-editor PRIVATE ${OLIVE_LIBRARIES})

app/timeline/timelineworkarea.cpp

@@ -106,17 +106,17 @@ void TimelineWorkArea::save(QXmlStreamWriter *writer) const
 							 QString::fromStdString(this->out().to_string()));
 }
 
-const Rational &TimelineWorkArea::in() const
+Rational TimelineWorkArea::in() const
 {
 	return workarea_range_.in();
 }
 
-const Rational &TimelineWorkArea::out() const
+Rational TimelineWorkArea::out() const
 {
 	return workarea_range_.out();
 }
 
-const Rational &TimelineWorkArea::length() const
+Rational TimelineWorkArea::length() const
 {
 	return workarea_range_.length();
 }

app/timeline/timelineworkarea.h

@@ -40,9 +40,9 @@ public:
 	bool enabled() const;
 	void set_enabled(bool e);
 
-	const Rational &in() const;
-	const Rational &out() const;
-	const Rational &length() const;
+	Rational in() const;
+	Rational out() const;
+	Rational length() const;
 	const TimeRange &range() const;
 	void set_range(const TimeRange &range);