Create the use-color sibling workspace

[Copilot]

Summary

• add a one-layer-deep Rust workspace for use-color
• implement use-rgb, use-hex-color, use-hsl, use-luminance, and use-contrast
• add per-crate READMEs, a root README, and CI for fmt, clippy, and tests

Validation

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• parallel_validation (Code Review + CodeQL)