feat(image-engine): add Sharp wrapper with 14 image operations

Operations: resize, crop, rotate, flip, convert, compress, strip-metadata,
brightness, contrast, saturation, color-channels, grayscale, sepia, invert.
Includes format detection, MIME mapping, metadata parsing, and engine orchestrator.

packages/image-engine/src/formats/detect.ts

@@ -0,0 +1,57 @@
+import sharp from "sharp";
+
+const MAGIC_BYTES: Array<{ bytes: number[]; offset: number; format: string }> = [
+  { bytes: [0x89, 0x50, 0x4e, 0x47], offset: 0, format: "png" },
+  { bytes: [0xff, 0xd8, 0xff], offset: 0, format: "jpeg" },
+  { bytes: [0x47, 0x49, 0x46, 0x38], offset: 0, format: "gif" },
+  { bytes: [0x52, 0x49, 0x46, 0x46], offset: 0, format: "webp" }, // RIFF header (check WEBP after)
+  { bytes: [0x49, 0x49, 0x2a, 0x00], offset: 0, format: "tiff" }, // Little-endian TIFF
+  { bytes: [0x4d, 0x4d, 0x00, 0x2a], offset: 0, format: "tiff" }, // Big-endian TIFF
+  { bytes: [0x42, 0x4d], offset: 0, format: "bmp" },
+];
+
+/**
+ * Detect the image format from a buffer.
+ * Uses Sharp metadata first, falls back to magic byte detection.
+ */
+export async function detectFormat(buffer: Buffer): Promise<string> {
+  try {
+    const metadata = await sharp(buffer).metadata();
+    if (metadata.format) {
+      return metadata.format;
+    }
+  } catch {
+    // Sharp couldn't parse it; fall through to magic bytes
+  }
+
+  return detectByMagicBytes(buffer);
+}
+
+function detectByMagicBytes(buffer: Buffer): string {
+  for (const entry of MAGIC_BYTES) {
+    if (buffer.length < entry.offset + entry.bytes.length) {
+      continue;
+    }
+
+    let match = true;
+    for (let i = 0; i < entry.bytes.length; i++) {
+      if (buffer[entry.offset + i] !== entry.bytes[i]) {
+        match = false;
+        break;
+      }
+    }
+
+    if (match) {
+      // For RIFF, verify it's actually WEBP
+      if (entry.format === "webp" && buffer.length >= 12) {
+        const webpSignature = buffer.slice(8, 12).toString("ascii");
+        if (webpSignature !== "WEBP") {
+          continue;
+        }
+      }
+      return entry.format;
+    }
+  }
+
+  return "unknown";
+}