fix: rgbToHsl no longer mutates its input array

[InukaWijerathna]

Closes#1907.

rgbToHsl aliased its argument (let colorHsl = colorRgb) instead of copying it, then wrote the HSL result into that same array and returned it. Any caller that kept a reference to its RGB input had that array silently overwritten with the HSL output (input === result was true).

Fixes the alias to a copy ([...colorRgb]), per the fix suggested in the issue, and adds a regression test asserting the input array is left unchanged after conversion.

The existing HSL conversion math is untouched — only the in-place mutation is fixed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.91%. Comparing base (5c39e87) to head (e1dc94f).

Additional details and impacted files

@@ Coverage Diff @@## master #1908 +/- ##
=======================================
Coverage 85.91% 85.91% =======================================
Files 379 379 Lines 19778 19778 Branches 3016 3015 -1 =======================================
Hits 16993 16993 Misses 2785 2785 

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