refactor(timeline): de-Qt oaktimeline and wrap it in a pure C ABI

- de-Qt all 11 sources: markers/workarea lose QObject/signals (list
  re-sort via direct resort() call, drawing moves to the app layer),
  explicit unique_ptr ownership for marker lists and viewer's
  workarea/markers
- timeline undo command families (split/pointer/ripple/general/track)
  now hold oaknode C handles and do all graph work through the oaknode
  C ABI (no adapter layer); gaps are attached to the project graph
  before insertion, orphan handles tracked and freed
- fix the de-Qt severed Block->Track length-changed chain with a
  direct block_length_changed() call (positions were stale after
  split/trim)
- expand oaknode C API by ~20 functions needed by timeline
  (copy_in_graph, block kind/casts, connect_element, input arrays,
  tracklist family, marker/workarea borrowed outlets)
- pure C ABI in include/timeline (marker/workarea/edit, command
  factories returning OakUndoCommand), oakcommon XML get_native
  accessors
- oaknode<->oaktimeline resolve each other at runtime; standalone
  drivers link both into test binaries
- tests: oaktimeline 117, regressions oakcommon 193 / oaknode 96 /
  oakrender 42 / oakcodec 18 / oakaudio 36 all green

include/common/xmlutils.h

@@ -64,6 +64,21 @@ typedef struct OakXmlWriter {
  */
 OakXmlReader oakcommon_xml_reader_init(const char *data);
 
+#ifdef __cplusplus
+} /* extern "C" */
+
+namespace olive { class XmlStreamReader; class XmlStreamWriter; }
+
+extern "C" {
+#endif
+
+/**
+ * @brief Borrowed access to the underlying C++ reader/writer (C++ only,
+ * for adapter layers). Valid while the handle is held. NULL-safe.
+ */
+olive::XmlStreamReader *oakcommon_xml_reader_get_native(OakXmlReader reader);
+olive::XmlStreamWriter *oakcommon_xml_writer_get_native(OakXmlWriter writer);
+
 /**
  * @brief Release one reference to a reader.
  *