fix: round Hex4/Hex8 alpha with ALPHA_PRECISION, not a hardcoded 2

[afonsojanu]

What

parseHex rounded the alpha channel of Hex4/Hex8 colors to 2 decimal places, while every other color model in the library (rgb, hsl, hsv, hwb, lab, lch, cmyk, xyz) rounds alpha to ALPHA_PRECISION (3). That constant was bumped from 2 to 3 a while back specifically because 2 digits weren't accurate enough (see the comment on it in constants.ts), but hex.ts still had the old value hardcoded in two places.

Since an alpha byte only has 256 possible values, I checked how many of them survive a round trip through a decimal string at each precision:

• precision 2: 155 out of 256 bytes don't round-trip
• precision 3 (ALPHA_PRECISION): 0 out of 256 fail

Concretely, this is issue #97:

colord("#cc88").toRgbString();
// "rgba(204, 204, 136, 0.53)"   <- alpha rounded to 2 digits

colord("rgba(204, 204, 136, 0.53)").toHex();
// "#cccc8887", not "#cccc8888"  <- 0x88 became 0x87

Fix

Both places in parseHex (Hex4 and Hex8) now round with ALPHA_PRECISION instead of a hardcoded 2, matching every other color model.

colord("#cc88").toRgbString();
// "rgba(204, 204, 136, 0.533)"

colord("rgba(204, 204, 136, 0.533)").toHex();
// "#cccc8888"  <- round-trips exactly now

Behavior change (called out per AGENTS.md)

This is a deliberate behavior change: parsing a Hex4/Hex8 string now resolves alpha to 3 decimal digits instead of 2 (e.g. #80 -> 0.502 instead of 0.5). toHex() output and the rounded RGB byte values are unchanged; only the decimal alpha exposed by toRgb()/toRgbString() and anything derived from it (lab/lch/cmyk/hwb conversions) gains a digit for alpha bytes that weren't exactly representable at 2 digits. I updated the existing test expectations affected by this (a handful of #...80/#...ed-style fixtures in tests/colord.test.ts and tests/plugins.test.ts) and added a dedicated round-trip regression test covering all 256 alpha bytes for both Hex4 and Hex8.

Testing

• npm run lint, npm run check-types, npm run test (103/103 passing, 100% line coverage), npm run size all pass locally.
• Added tests/colord.test.ts: "Round-trips every Hex4/Hex8 alpha byte through RGB without drift".

Closes #97.

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