Implement PDF color operators (g/G, rg/RG, k/K, cs/CS, sc/SC/scn/SCN) that
populate fill_color and stroke_color fields in GraphicsState.
Changes:
- Add ColorSpace enum with all PDF color space variants
- Add fill_color_space and stroke_color_space tracking to GraphicsState
- Implement color-setting methods for all operator types
- Add parse_color_space() helper to content_stream.rs
- Implement color operator parsing in content_stream match statement
- Add 24 acceptance criteria tests
Closes: pdftract-4ubed
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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