fix(hcg): wrap negative hue into [0, 360) for rgb -> hcg

[spokodev]

Problem

rgb.hcg can return a hue outside its declared [0, 360) range. When red is the maximum channel and blue > green, the red sextant branch computes a negative value:

if (max === r) {
    hue = ((g - b) / chroma % 6); // negative when b > g
}
...
hue = (hue % 1); // JS modulo is sign-preserving, so this stays negative

hcg declares min: [0, 0, 0], max: [360, 100, 100], so a negative hue violates the space's own range, and it breaks the rgb -> hcg -> rgb round-trip:

const space = require('color-space');

space.rgb.hcg([255, 0, 55]);
// actual:   [ -12.94, 100, 0 ]   <- hue out of range
// expected: [ 347.06, 100, 0 ]

space.hcg.rgb(space.rgb.hcg([255, 0, 55]));
// actual:   [ 255, 0, 310 ]      <- blue channel impossible
// expected: [ 255, 0, 55  ]

The sibling spaces agree the hue should be ~347: rgb.hsi([255, 0, 55]) is 348.17 and rgb.hsp([255, 0, 55]) is 347. Only hcg returns a negative.

This affects the whole red-dominant, blue > green region of the RGB cube.

Fix

Add the same negative-hue wrap that rgb.hsv and rgb.hsl already use (if (h < 0) { h += ... }), applied here in the fractional domain before scaling to degrees:

hue = (hue % 1);
if (hue < 0) {
    hue += 1;
}

Tests

Extended the existing hcg: rgb -> hcg test with the red-dominant/blue > green case and a round-trip assertion. Both fail on master and pass with the fix. Full suite: 1353 pass, 0 fail.

[dy]

Thanks @spokodev — this is a real bug and a clean fix. v3 already lands the identical fix: hcg.js wraps negative hues with if (hue < 0) hue += 1, and your exact case rgb [255,0,55] → hcg [347,100,0] passes (0 out-of-range hues across a full RGB sweep). I'm porting your targeted test assertion into the v3 hcg test as a regression pin, with credit. Since v3 (#67) is a full rewrite that supersedes master, this PR will be closed when v3 merges — but the fix and your test live on. 🙏

[spokodev]

Thanks, glad it helped, and thanks for porting the test into v3 with credit. Fine to close this once v3 lands.