Add ColorBlending module for multi-image blending support

[izkgao]

Description

Closes #160.

Need frontend PR #2759 to be merged first.

This PR adds color blending support and refactors the session/image APIs around a shared "image-view item" model so frame-backed images and color blendings can be handled consistently.

What is implemented

• Add a new ColorBlending API with Layer objects for:
  • listing, adding, replacing, deleting, and reordering layers
  • preserving per-layer alpha during reordering/subsetting
  • setting layer alpha and colormaps
  • toggling raster/contour/vector visibility
• Introduce ImageBase as the shared abstraction for image-view items and move common behaviors such as make_active() and image_view_order onto the base class
• Update Image to align with the image-view-item model:
  • use file_id terminology consistently
  • expose richer __repr__ output
  • share activation and image-list-order behavior through ImageBase
• Extend Session with heterogeneous image-view-item support:
  • image_list() now returns both Image and ColorBlending
  • add _find_image_view_order()
  • replace the old image lookup helper with keyword-only image_by_id(image_view_order=..., file_id=..., color_blending_id=...)
  • replace active_frame() with active_image()
  • add create_color_blending() and open_as_color_blending()
  • add cached carta_version and include it in Session.__repr__ when available
  • update set_cursor() to use a better API
• Add new constants for ImageType and ColormapSet
• Update documentation and quickstart examples to cover:
  • heterogeneous image lists
  • image-view order
  • color blending workflows

Checklist

• Add code examples in documentation
• Add tests