fix(il2cpp): require FAT container identity

src/il2cpp/Il2cppBinaryImage.ts

@@ -1,4 +1,5 @@
 import {
+  classifyFatContainerEvidence,
   readFatContainerHeader,
   validateFatSlice,
 } from "../domain/apple/machoContainer.js";
@@ -34,7 +35,16 @@ const selectImage = (
   architecture: string | undefined,
 ): Buffer => {
   const fat = readFatContainerHeader(bytes, bytes.length);
-  if (fat.state === "not-fat") {
+  const identity =
+    fat.state === "parsed"
+      ? classifyFatContainerEvidence(fat, bytes)
+      : undefined;
+  if (identity?.state === "malformed")
+    throw new Il2cppBinaryFormatFailure("malformed-container", identity.reason);
+  if (
+    fat.state === "not-fat" ||
+    (fat.state === "parsed" && identity?.state !== "mach-o")
+  ) {
     if (architecture !== undefined)
       throw new Il2cppBinaryFormatFailure(
         "slice-not-found",

tests/boundary/il2cpp/il2cppBinaryFile.test.ts

@@ -1,3 +1,4 @@
+import { javaClassFile } from "../../fixtures/javaClassFile.js";
 import { Il2cppBinaryFormatFailure } from "../../../src/il2cpp/Il2cppBinaryFormat.js";
 import { readBinaryContainer } from "../../../src/native/metadata/readBinaryContainer.js";
 import {
@@ -191,10 +192,33 @@ it("rejects reused MethodDef tokens within an image but preserves cross-image re
   );
 });
 
+it.each([0, 4000, 9000])(
+  "reports a valid JVM class with %i unused constant bytes as unsupported",
+  (constantBytes) => {
+    expect(failure(javaClassFile(constantBytes, true))).toEqual({
+      reason: "unsupported-container",
+      kind: "unsupported",
+    });
+  },
+);
+
+it("does not infer universal Mach-O identity from a FAT-shaped non-Mach-O table", () => {
+  // CAFEBABE also identifies JVM classes. A plausible FAT table and known CPU
+  // do not prove Mach-O unless the declared slice contains a Mach-O header.
+  const bytes = createUniversalFixture([
+    { cpu: 0x0100000c, bytes: Buffer.alloc(0x100) },
+  ]);
+  expect(failure(bytes)).toEqual({
+    reason: "unsupported-container",
+    kind: "unsupported",
+  });
+  expect(failure(bytes, "arm64").reason).toBe("slice-not-found");
+});
+
 it("requires an explicit slice of a universal Mach-O and reads only that slice", () => {
   const thin = createIl2cppBinaryFixture();
   const universal = createUniversalFixture([
-    { cpu: 0x01000007, bytes: Buffer.alloc(0x100) },
+    { cpu: 0x01000007, bytes: createIl2cppBinaryFixture() },
     { cpu: 0x0100000c, bytes: thin },
   ]);
   expect(failure(universal)).toEqual({