Support lab(), lch(), oklab(), and oklch() colors in the native CSS parser

[ya-nsh]

Summary:

The Fabric CSS color parser supports rgb(), hsl(), and hwb(), and has a TODO T213000437 for the rest of the CSS Color 4 color functions. This PR implements four of them: lab(), lch(), oklab(), and oklch().

These are increasingly how colors are authored on the web. Tailwind CSS v4 defines its whole default palette in oklch(), and design tokens exported for the web are often in OKLCH. Today those strings don't parse in React Native, so teams sharing tokens between web and native need a conversion step. This was requested in discussions-and-proposals#871.

This change is limited to the native CSS parser (enableNativeCSSParsing). The JS normalize-color path is unchanged. I'm happy to follow up with a matching JS change if maintainers want one.

Behavior

Everything follows CSS Color 4:

• Syntax: modern space-separated syntax only, with an optional / alpha. These functions have no legacy comma syntax, so commas are rejected, as is an alpha that isn't preceded by /.
• Percentages:
  • Lightness: 100% is 100 for lab/lch and 1 for oklab/oklch.
  • a/b: 100% is 125 for lab and 0.4 for oklab.
  • Chroma: 100% is 150 for lch and 0.4 for oklch.
• Hue: a number (degrees) or any <angle> unit (deg, rad, grad, turn); out-of-range values wrap.
• none: accepted for any component, including alpha, and resolves to zero (missing components).
• Parse-time clamping: lightness is clamped to [0%, 100%], and negative chroma is clamped to 0.
• Conversion: lab()/lch() use the D50 white point with Bradford adaptation to D65. All conversions use the spec's sample-code matrices, computed in double precision.
• Gamut mapping: colors outside sRGB use the spec's CSS gamut mapping algorithm, which reduces OKLCh chroma (JND 0.02) while preserving lightness and hue, instead of clipping each channel. Clipping noticeably shifts vivid colors. For example, the orange oklab(0.7 0.4 0.4) clips to rgb(255, 0, 0) but gamut-maps to rgb(255, 97, 0). Many Tailwind v4 colors are outside sRGB, so this affects real palettes.
• Robustness: a component that becomes non-finite (for example, a huge rad hue that overflows when converted to degrees) makes the value invalid instead of reaching an undefined float-to-int conversion. The gamut-mapping binary search is bounded, so extreme chroma values still converge.

Hue and alpha handling of the existing rgb()/hsl()/hwb() functions is unchanged. color() and color-mix() remain in the TODO.

Changelog:

[GENERAL] [ADDED] - Support lab(), lch(), oklab(), and oklch() colors in the native CSS parser

Test Plan:

Unit tests: added 10 tests to CSSColorTest.cpp, covering each function, percentages, every angle unit, hue wrapping, alpha, none components, lightness and chroma clamping, gamut mapping, and 25 invalid inputs. Expected values come from ColorAide, whose minde-chroma fit implements the spec's gamut-mapping algorithm.

Open-source CI doesn't build the react/renderer/css gtests, so I compiled them directly with React Native's compiler flags (-std=c++20 -Wall -Werror -Wpedantic -fexceptions -frtti) against googletest 1.15.2 and fast_float 8.0.0 (the version React Native pins):

[==========] 328 tests from 19 test suites ran.
[  PASSED  ] 328 tests.

That's all 318 existing CSS parser tests plus the 10 new ones.

Differential testing against the reference implementation: I built a small CLI around parseCSSProperty<CSSColor> and compared it with ColorAide:

• 83,000 random lab/lch/oklab/oklch colors, biased toward vivid and extreme-lightness values so about 93% need gamut mapping: 82,999 exact 8-bit matches. The one off-by-one comes from CSSNumber storing values as float. When ColorAide gets the same float32-rounded input, it matches exactly.
• 40,000 random sRGB colors written as each of the four functions: all 40,000 round-trip to the original 8-bit sRGB value.

Sanitizers: the color tests, the differential runs above, and inputs near FLT_MAX (for example, lch(50 3e38 3e38) and oklch(0.7 0.1 1e38rad)) all pass under -fsanitize=undefined,float-cast-overflow,float-divide-by-zero. I couldn't run AddressSanitizer: its runtime hangs during startup on my machine (macOS 27).

Formatting and API snapshots:

• Formatted with the pinned clang-format 21.1.2.
• Regenerated the C++ API snapshots with Doxygen 1.16.1, the version CI uses: python -m scripts.cxx-api.parser.

Not run: Fantom and RNTester. This change is limited to the parser, which runs only when enableNativeCSSParsing is enabled.