feat(worker): 7z signature-header parser

worker/src/archive/ranged/sevenz-ranged.ts

@@ -0,0 +1,12 @@
+const SEVENZ_MAGIC = Buffer.from([0x37, 0x7a, 0xbc, 0xaf, 0x27, 0x1c]);
+
+export function parseSevenZSignatureHeader(
+  buf: Buffer,
+): { nextHeaderOffset: number; nextHeaderSize: number } | null {
+  if (buf.length < 32) return null;
+  if (!buf.subarray(0, 6).equals(SEVENZ_MAGIC)) return null;
+  return {
+    nextHeaderOffset: Number(buf.readBigUInt64LE(12)),
+    nextHeaderSize: Number(buf.readBigUInt64LE(20)),
+  };
+}