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/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();
 }