feat: add image converter family (sharp)

Regenerates the sample.png test fixture via sharp itself: the
hand-encoded base64 PNG from the plan had valid magic bytes (enough to
fool file-type's signature check) but was malformed past the header,
which libpng rejected as soon as sharp actually tried to decode it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/converters/image.js

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+import sharp from 'sharp';
+import { register } from './registry.js';
+
+const IMAGE_FORMATS = ['jpg', 'jpeg', 'png', 'webp', 'gif', 'tiff', 'avif'];
+
+function sharpFormatName(format) {
+  return format === 'jpg' ? 'jpeg' : format;
+}
+
+export function registerImageConverters() {
+  for (const sourceFormat of IMAGE_FORMATS) {
+    for (const targetFormat of IMAGE_FORMATS) {
+      if (sourceFormat === targetFormat) continue;
+
+      register({
+        family: 'image',
+        sourceFormat,
+        targetFormat,
+        convert: async (inputPath, outputPath) => {
+          await sharp(inputPath).toFormat(sharpFormatName(targetFormat)).toFile(outputPath);
+        },
+      });
+    }
+  }
+}

test/converters/image.test.js

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+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import fs from 'node:fs/promises';
+import path from 'node:path';
+import os from 'node:os';
+import { registerImageConverters } from '../../src/converters/image.js';
+import { resolve, listTargetFormats } from '../../src/converters/registry.js';
+import { detectInputMime } from '../../src/mime.js';
+
+let tmpDir;
+
+beforeAll(async () => {
+  registerImageConverters();
+  tmpDir = await fs.mkdtemp(path.join(os.tmpdir(), 'converter-image-'));
+});
+
+afterAll(async () => {
+  await fs.rm(tmpDir, { recursive: true, force: true });
+});
+
+describe('image converters', () => {
+  it('registers every pair among the supported formats', () => {
+    const targets = listTargetFormats('png').sort();
+    expect(targets).toEqual(['avif', 'gif', 'jpeg', 'jpg', 'tiff', 'webp']);
+  });
+
+  it('converts a PNG fixture to WebP', async () => {
+    const inputPath = path.join(import.meta.dirname, '..', 'fixtures', 'sample.png');
+    const outputPath = path.join(tmpDir, 'output.webp');
+    const entry = resolve('png', 'webp');
+
+    await entry.convert(inputPath, outputPath);
+
+    const detected = await detectInputMime(outputPath);
+    expect(detected.mime).toBe('image/webp');
+  });
+
+  it('converts a PNG fixture to JPG', async () => {
+    const inputPath = path.join(import.meta.dirname, '..', 'fixtures', 'sample.png');
+    const outputPath = path.join(tmpDir, 'output.jpg');
+    const entry = resolve('png', 'jpg');
+
+    await entry.convert(inputPath, outputPath);
+
+    const detected = await detectInputMime(outputPath);
+    expect(detected.mime).toBe('image/jpeg');
+  });
+});