fix(hwb): normalize whiteness + blackness over 100% to gray

[spokodev]

Problem

When an HWB color's whiteness + blackness is >= 100%, the result should be an achromatic gray, but the HWB plugin produces wrong colors:

colord({ h: 120, w: 70, b: 70 }).toHex(); // "#b34db3" (purple)   expected "#808080"
colord({ h: 200, w: 100, b: 100 }).toHex(); // "#000000" (black)  expected "#808080"
colord("hwb(0 50% 60%)").toHex();          // "#668080"           expected "#747474"

These are valid CSS inputs that every browser renders as gray.

Cause

hwbaToRgba converted via HSV without the w + b >= 100% normalization:

• For w + b > 100, 100 - (w / (100 - b)) * 100 is negative, feeding a negative saturation into HSV → garbage.
• The b === 100 special case returned s: 0, v: 0 (black) instead of gray.

Fix

Per CSS Color 4 §8.1, when white + black >= 100% the color is gray with value white / (white + black):

if (hwba.w + hwba.b >= 100) {
  const gray = (hwba.w / (hwba.w + hwba.b)) * 255;
  return { r: gray, g: gray, b: gray, a: hwba.a };
}

The b === 100 special case is removed because every b === 100 input has w + b >= 100 and is now handled by the new branch. At the exact w + b === 100 boundary the new branch yields the same value the old HSV path did, so nothing else changes.

Verification

• New test in tests/plugins.test.ts; fails before, passes after. Full suite: 78 passing.
• Differential vs the CSS Color 4 algorithm (pure-hue HSL mix, independent of the HSV path used here) over a 15°×5%×5% grid plus 500,000 random inputs: 0 divergences (>1 LSB). This also confirms the untouched w + b < 100 path already matches the spec.