feat(importer): phase 5 — color correction sliders

Brightness, contrast, saturation, and gamma adjustments before palette
quantization. Pure-function applyColorCorrection in src/lib keeps the CLI
and browser importer in sync. Inserted post-resize so slider moves stay
responsive on large sources; precision loss is negligible through a
32-color palette.

UI is a collapsible "Color correction" section with four sliders, live
values, a "Reset" button, and an "on" badge when any value differs from
default. CLI gains --brightness, --contrast, --saturation, --gamma
(all -100..+100 except gamma which is 0.1..3.0).

9 unit tests pin down identity, saturate-to-0-or-255 clamping, greyscale
at saturation=-100, gamma midtone direction, and alpha preservation.

test/lib/image-color-correction.test.js

@@ -0,0 +1,74 @@
+import { describe, it, expect } from 'vitest';
+import { applyColorCorrection } from '../../src/lib/image-color-correction.js';
+
+function solid(w, h, r, g, b, a = 255) {
+  const out = new Uint8ClampedArray(w * h * 4);
+  for (let i = 0; i < w * h; i++) {
+    out[i * 4] = r; out[i * 4 + 1] = g; out[i * 4 + 2] = b; out[i * 4 + 3] = a;
+  }
+  return out;
+}
+
+describe('applyColorCorrection', () => {
+  it('identity (all defaults) returns a copy of the input', () => {
+    const src = solid(2, 2, 100, 150, 200, 220);
+    const out = applyColorCorrection(src, 2, 2);
+    expect(out).toEqual(src);
+    expect(out).not.toBe(src);
+  });
+
+  it('brightness +100 clamps grey pixels to 255', () => {
+    const src = solid(1, 1, 128, 128, 128);
+    const out = applyColorCorrection(src, 1, 1, { brightness: 100 });
+    // 128 + 255 = 383, clamped to 255
+    expect([out[0], out[1], out[2]]).toEqual([255, 255, 255]);
+  });
+
+  it('brightness -100 clamps grey pixels to 0', () => {
+    const src = solid(1, 1, 128, 128, 128);
+    const out = applyColorCorrection(src, 1, 1, { brightness: -100 });
+    expect([out[0], out[1], out[2]]).toEqual([0, 0, 0]);
+  });
+
+  it('contrast +100 pushes dark darker and bright brighter', () => {
+    const dark = solid(1, 1, 50, 50, 50);
+    const bright = solid(1, 1, 200, 200, 200);
+    const dOut = applyColorCorrection(dark, 1, 1, { contrast: 100 });
+    const bOut = applyColorCorrection(bright, 1, 1, { contrast: 100 });
+    expect(dOut[0]).toBeLessThan(50);
+    expect(bOut[0]).toBeGreaterThan(200);
+  });
+
+  it('contrast 0 is identity (within rounding)', () => {
+    const src = solid(1, 1, 100, 100, 100);
+    const out = applyColorCorrection(src, 1, 1, { contrast: 0 });
+    expect([out[0], out[1], out[2]]).toEqual([100, 100, 100]);
+  });
+
+  it('saturation -100 produces greyscale (R=G=B)', () => {
+    const src = solid(1, 1, 200, 80, 30);
+    const out = applyColorCorrection(src, 1, 1, { saturation: -100 });
+    expect(out[0]).toBe(out[1]);
+    expect(out[1]).toBe(out[2]);
+  });
+
+  it('gamma 1 is identity; gamma > 1 brightens midtones', () => {
+    const src = solid(1, 1, 128, 128, 128);
+    const id = applyColorCorrection(src, 1, 1, { gamma: 1 });
+    expect(id[0]).toBe(128);
+    const bright = applyColorCorrection(src, 1, 1, { gamma: 2.2 });
+    expect(bright[0]).toBeGreaterThan(128);
+  });
+
+  it('gamma < 1 darkens midtones', () => {
+    const src = solid(1, 1, 128, 128, 128);
+    const out = applyColorCorrection(src, 1, 1, { gamma: 0.5 });
+    expect(out[0]).toBeLessThan(128);
+  });
+
+  it('alpha channel is preserved unchanged', () => {
+    const src = solid(1, 1, 50, 100, 150, 77);
+    const out = applyColorCorrection(src, 1, 1, { brightness: 50, contrast: 50, saturation: 50, gamma: 2.2 });
+    expect(out[3]).toBe(77);
+  });
+});