Fix Retina JPEG scaling for stream-video

[EuanTop]

Summary

Fixes #83.

This updates JPEG frame scaling to produce the requested output pixel dimensions regardless of the host display backing scale. Previously, the macOS NSImage.lockFocus() path could render through the Retina backing scale, so --scale 0.5 could still emit frames at the original Retina-sized dimensions.

The new path decodes to CGImage, computes exact scaled dimensions, draws into an explicitly sized CGContext, and encodes the result with CGImageDestination.

Changes

• Replace the AppKit-based JPEG scale/re-encode path with ImageIO/CoreGraphics encoding.
• Add an explicit JPEG encode failure error.
• Add tests that assert scaled JPEG output has exact pixel dimensions.
• Add coverage that quality-only re-encoding preserves dimensions and changes output size.

Validation

• make build
• swift test --filter VideoFrameProcessingTests
• git diff --check
• Live simulator verification:
  • --scale 1.0: 1206x2622
  • --scale 0.5: 602x1310

[onevcat]

LGTM! Thank you for this.