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

core/src/util/bezier.cpp

@@ -23,7 +23,7 @@
 
 #include <algorithm>
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 Bezier::Bezier()

core/src/util/color.cpp

@@ -27,7 +27,7 @@
 #include <math.h>
 #include <stdint.h>
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 Color Color::from_hsv(const DataType &h, const DataType &s, const DataType &v)

core/src/util/fractionutils.cpp

@@ -27,7 +27,7 @@
 
 #include <limits>
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 namespace

core/src/util/rational.cpp

@@ -31,7 +31,7 @@
 #include "util/fractionutils.h"
 #include "util/stringutils.h"
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 const Rational Rational::na_n = Rational(0, 0);

core/src/util/stringutils.cpp

@@ -24,7 +24,7 @@
 #include <stdarg.h>
 #include <stdexcept>
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 std::vector<std::string> StringUtils::split(const std::string &s,

core/src/util/timecodefunctions.cpp

@@ -27,7 +27,7 @@
 #include "util/fractionutils.h"
 #include "util/stringutils.h"
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 std::string Timecode::time_to_timecode(const Rational &time,

core/src/util/timerange.cpp

@@ -27,7 +27,7 @@
 
 #include "util/timecodefunctions.h"
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 TimeRange::TimeRange(const Rational &in, const Rational &out)

core/src/util/value.cpp

@@ -21,7 +21,7 @@
 
 #include "util/value.h"
 
-namespace olive::core
+namespace olive::core::internal
 {
 
 }