v0.2.0

Changed

• Crate layering. oxframe-core now owns Image (RGBA pixels) and the new
  FrameSource trait — "give me frame n" — through which the editor
  (oxframe-preview), render/snapshot and the exporter get frames
  without knowing what draws them. oxframe-preview and oxframe-export no
  longer depend on the 2D crate or the renderers; CI checks this
  (scripts/check-layering.sh). Scene code is unchanged and renders
  byte-identically.
• The crate docs and README carry a development-status notice.

Breaking (only for code using the internal crates directly)

• oxframe_preview::PreviewEngine now extends oxframe_core::FrameSource:
  fps, size, total_frames and render moved to that trait.

  // before
  impl PreviewEngine for MyEngine { fn fps(&self) -> u32 { 60 } /* … */ }
  // after
  impl FrameSource for MyEngine { fn fps(&self) -> u32 { 60 } /* … */ }
  impl PreviewEngine for MyEngine { /* the editor-specific methods */ }

• oxframe_render::Image is a re-export of oxframe_core::Image;
  Image::encode_png still exists.