fix(plugin-color): compute sepia output channels from the original pixel

[spokodev]

sepia() returns wrong colors for any non-grayscale pixel. The sepia matrix needs each output channel to be a linear combination of the original r, g, b, but the code overwrites red first, then computes green from the new red, then blue from the new red and green:

red = red * 0.393 + green * 0.769 + blue * 0.189;
green = red * 0.349 + green * 0.686 + blue * 0.168;   // reads the new red
blue = red * 0.272 + green * 0.534 + blue * 0.131;    // reads the new red and green

For a pure red pixel (255, 0, 0) the correct sepia output is (100, 88, 69); the current code returns (100, 34, 45).

This is a regression: the same bug was reported (#346) and fixed (#483) in 2018, and the v1 TypeScript rewrite reintroduced it.

Fix

Hold the original channels and derive each output from them, matching the merged 2018 fix.

Testing

Added a test that drives sepia against a known red pixel and asserts (100, 88, 69). It fails before this change. The existing packages/jimp sepia snapshot had captured the buggy output (64-22-2D); it is rebuilt to the corrected values (64-58-45). Full plugin-color suite stays green.