Preserve alpha when setting color channels

[jakezwang]

Changing a color channel can discard opacity: chroma('red').alpha(0.4).set('rgb.r', 128).alpha() returns 1. RGB, HSV, HSI, and CMYK conversions omit alpha, so constructing the result makes it opaque.

Preserve the original alpha after converting the edited channels, as darken() and saturate() already do. Tests cover transparent, translucent, and opaque colors, both mutation modes, relative changes, and HSL/Lab controls. Includes a patch changeset.

Related to the opacity-loss report in #244; this does not add the separate rgba.a feature requested there.

Validation: all 2,569 tests pass on Node 20, 24, and 26; lint and all four distribution builds pass.

[changeset-bot]

🦋 Changeset detected

Latest commit: aa6e9ea

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
chroma-js	Patch

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