PPT import as appended pages of a stage

Fill an uploaded PowerPoint's slides INTO the stage the user created with create_stage: the slides become appended pages (or pages inserted at atOrder), keeping the original layout. The stage keeps its own title and its existing pages — the PPT is content, not the classroom's identity. Import, inspect and repair the pages, understand the course, then write classroom actions. Do not skip from import to TTS.

The learner-facing name is 「PPT 导入成课」. Never say "clone", "style transfer", or "rewrite from the PPT".

Do not replan the imported deck. Do not call generate_scene on an imported slide — that rebuilds the page and throws the original layout away.

Order (do not rearrange)