Clip cover-mode designs to the placement area

[SLP-DEV1]

Summary

Makes flat-image cover / Fill Slot behave like a real cover crop in both export and the visual editor.

Changes

• clip cover-mode Canvas rendering to the target rectangle before applying rotation
• keep contain/width/height behavior unchanged
• add a separate clipped artwork layer in the visual editor while keeping transform handles usable
• add unit coverage for cover clipping and non-cover behavior

Closes #16