feat: use Manim's next_section for subsections

When next_subsection() is called, it now also calls Manim's next_section()
internally. This allows Manim to create separate video files for each
subsection automatically when --save_sections flag is used.

This is the first step toward eliminating ffmpeg post-processing and
letting Manim do the video splitting work natively.

Author: krystophny

manim_slides/slide/base.py

@@ -340,6 +340,10 @@ def construct(self):
         )
         self._pending_subsection_markers.append(marker)
 
+        # Call Manim's next_section to create separate video files
+        if hasattr(self, 'next_section'):
+            self.next_section(name=name or f"subsection_{len(self._pending_subsection_markers)}")
+
     def _consume_subsection_markers(self) -> tuple[SubsectionMarker, ...]:
         markers = tuple(self._pending_subsection_markers)
         self._pending_subsection_markers.clear()