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_test.go

@@ -0,0 +1,68 @@
+package colorhash
+
+import (
+	"math"
+	"testing"
+)
+
+func TestGenerateOKLCHPalette(t *testing.T) {
+	palette := GenerateOKLCHPalette(8, 0.7, 0.15)
+
+	if len(palette) != 8 {
+		t.Fatalf("expected 8 colors, got %d", len(palette))
+	}
+
+	// All colors should have approximately the same lightness and chroma
+	for i, c := range palette {
+		oklch := c.ToOKLCH()
+		if math.Abs(oklch.L-0.7) > 0.05 {
+			t.Errorf("color %d: L = %f, expected ~0.7", i, oklch.L)
+		}
+		if math.Abs(oklch.C-0.15) > 0.03 {
+			t.Errorf("color %d: C = %f, expected ~0.15", i, oklch.C)
+		}
+	}
+
+	// Hues should be evenly spaced (45° apart for 8 colors)
+	for i, c := range palette {
+		oklch := c.ToOKLCH()
+		expectedH := float64(i) * 45.0
+		diff := math.Abs(oklch.H - expectedH)
+		if diff > 180 {
+			diff = 360 - diff
+		}
+		if diff > 10.0 {
+			t.Errorf("color %d: H = %f, expected ~%f", i, oklch.H, expectedH)
+		}
+	}
+}
+
+func TestGenerateOKLCHPaletteEmpty(t *testing.T) {
+	palette := GenerateOKLCHPalette(0, 0.7, 0.15)
+	if len(palette) != 0 {
+		t.Errorf("expected empty palette, got %d colors", len(palette))
+	}
+}
+
+func TestGenerateOKLCHPaletteSingle(t *testing.T) {
+	palette := GenerateOKLCHPalette(1, 0.6, 0.1)
+	if len(palette) != 1 {
+		t.Fatalf("expected 1 color, got %d", len(palette))
+	}
+	oklch := palette[0].ToOKLCH()
+	if oklch.H > 5.0 && oklch.H < 355.0 {
+		t.Errorf("single color H = %f, expected ~0", oklch.H)
+	}
+}
+
+func TestGenerateOKLCHPaletteDistinct(t *testing.T) {
+	palette := GenerateOKLCHPalette(6, 0.7, 0.15)
+	// All colors should be distinct
+	seen := make(map[int]bool)
+	for _, c := range palette {
+		if seen[int(c)] {
+			t.Errorf("duplicate color: %s", c.ToHex())
+		}
+		seen[int(c)] = true
+	}
+}