fix(color): support libc++ experimental optional

[cristim]

Summary

• Use standard std::optional when available and std::experimental::optional with older libc++ headers.
• Keep the fallback within the OpenRenderBox color-space API and use operations supported by both implementations.

Verification

• Built ColorSpace.cpp as an arm64 macOS Mach-O object using Darling's older libc++ headers and current CoreGraphics declarations.
• Built the OpenRenderBox and OpenRenderBoxShims arm64 macOS Swift modules with Swift C++ interoperability.
• git diff --check passes.

This allows an older Darwin libc++ header set to compile this component. It does not provide missing CoreGraphics APIs or complete the full RenderBox runtime build.