Scratch.swift's channel() was marked deprecated ('Inefficient') and produced 3
warnings; it also force-unwrapped and built a CIContext per call.

- Add CGImage.extracting(_ channel:) in CGImage+ColorMatrix.swift, using the
  shared non-color-managed CIColorMatrix helper (now general row vectors) and
  throwing on failure. Remove channel()/redChannel/greenChannel/blueChannel.
- Update the SceneKit and RealityKit generators; drop an unused CoreImage import.

Test: ChannelExtractionTests (exact per-channel gray from a raw-byte pixel).
Old and new outputs agree (an apparent difference was a test-image artifact:
filling with an sRGB CGColor converts the bytes). Goldens unchanged.
Package build now has zero warnings.

Closes #54.