fix: correct luminance coefficients in cie() function

[godfengliang]

Fixes #77

The luminance calculation in had two issues:

1. Typo: The red channel coefficient was instead of the correct (per Rec. 709 / ITU-R BT.709)
2. Channel swap: Red and blue channels were swapped in the multiplication

Before:

This weighted blue ~7x higher than it should.

After:

Now correctly uses Rec. 709 coefficients: .

Reference: https://en.wikipedia.org/wiki/Rec._709