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

.github/workflows/cd.yml

@@ -67,10 +67,12 @@ jobs:
           # The FFmpeg bridge DLL is built outside app/ and is not reachable
           # via PATH, so ntldd cannot discover it from the executables alone
           cp build/ffmpeg_bridge/bin/ffmpeg_bridge.dll app/packaging/windows/nsis/oak-editor/
+          # Same for the liboakcore DLL
+          cp build/core/oakcore.dll app/packaging/windows/nsis/oak-editor/
           windeployqt6 app/packaging/windows/nsis/oak-editor/oak-editor.exe
           # Copy all non-Qt MSYS2 DLLs recursively for every binary we ship
           cd app/packaging/windows/nsis/oak-editor
-          for binary in oak-editor.exe oak-render-worker.exe oakgl.dll oakvulkan.dll ffmpeg_bridge.dll; do
+          for binary in oak-editor.exe oak-render-worker.exe oakgl.dll oakvulkan.dll ffmpeg_bridge.dll oakcore.dll; do
             [ -f "$binary" ] || continue
             for l in $(ntldd -R "$binary" | grep -E 'mingw64|ucrt64|clang64' | sed 's/^[ \t]*//' | cut -d' ' -f3); do
               cp -v "$l" .
@@ -369,9 +371,11 @@ jobs:
           ls AppDir/usr/lib/liboakvulkan.so                  || (echo "Missing liboakvulkan.so" && exit 1)
           ls AppDir/usr/lib/libvulkan.so*                    || (echo "Missing libvulkan" && exit 1)
           ls AppDir/usr/lib/libffmpeg_bridge.so              || (echo "Missing libffmpeg_bridge.so" && exit 1)
+          ls AppDir/usr/lib/liboakcore.so                    || (echo "Missing liboakcore.so" && exit 1)
           # Ensure every shipped binary resolves all of its dependencies
           ! ldd AppDir/usr/bin/oak-editor | grep "not found"
           ! ldd AppDir/usr/bin/oak-render-worker | grep "not found"
+          ! ldd AppDir/usr/lib/liboakcore.so | grep "not found"
           ! ldd AppDir/usr/lib/liboakgl.so | grep "not found"
           ! ldd AppDir/usr/lib/liboakvulkan.so | grep "not found"