Report second-order jack-knife bound instead of Chao

Author: CyberiaResurrection

src/hypofuzz/hy.py

@@ -404,11 +404,9 @@ def _json_description(self) -> Report:
             singletons = self.pool.singletons
             doubletons = self.pool.doubletons
             offset = (
-                singletons * (singletons - 1) / 2
-                if 0 == doubletons
-                else singletons * singletons / (2 * doubletons)
+                2 * singletons - doubletons
             )
-            offset = int(offset) + 1
+            offset = max(1, offset)
             report["est. branches"] = int(str(report["branches"])) + offset
         return report