feat: add OKLCH-aware palette generation (#5)

* chore: update deps, fix escape codes, expand tests, add README

- Bump Go 1.18 → 1.26, simplecolorpalettes v0.9.5 → v0.9.7
- Fix swapped fg/bg ANSI escape code values in trueColorString
- Expand test suite from 2 to 13 tests (determinism, positivity, color assignment, background contrast, stringer palette)
- Add README with usage examples

* feat: add OKLCH-aware palette generation

Add GenerateOKLCHPalette() which creates n evenly-spaced colors in the
OKLCH color space at a given lightness and chroma. This produces
perceptually uniform palettes where all colors appear equally bright.

Uses the new OKLCH support from simplecolorpalettes.

Note: go.mod contains a replace directive for local development that
must be removed before merge (after simplecolorpalettes is published).

* build: update simplecolorpalettes to v0.9.8 (charmtone palette)

oklch.go

@@ -0,0 +1,21 @@
+package colorhash
+
+import (
+	"github.com/taigrr/simplecolorpalettes/simplecolor"
+)
+
+// GenerateOKLCHPalette generates n evenly-spaced colors in the OKLCH color space
+// at the given lightness and chroma. This produces a perceptually uniform palette
+// where all colors appear equally bright and saturated.
+func GenerateOKLCHPalette(n int, l, c float64) simplecolor.SimplePalette {
+	if n <= 0 {
+		return simplecolor.SimplePalette{}
+	}
+	palette := make(simplecolor.SimplePalette, n)
+	step := 360.0 / float64(n)
+	for i := 0; i < n; i++ {
+		h := float64(i) * step
+		palette[i] = simplecolor.FromOKLCH(l, c, h)
+	}
+	return palette
+}