fix(il2cpp): preserve recognized truncated metadata prefixes

tests/boundary/il2cpp/il2cppMetadataFile.test.ts

@@ -7,6 +7,7 @@ import {
   il2cppIndexWidth,
   il2cppMetadataRecordBytes,
   readIl2cppMetadata,
+  readIl2cppMetadataForLinking,
   readIl2cppMetadataSteps,
 } from "../../../src/il2cpp/Il2cppMetadataFile.js";
 import {
@@ -24,9 +25,14 @@ import {
   type Il2cppMetadataFixture,
 } from "../../fixtures/il2cpp/metadata.js";
 
-const failure = (bytes: Buffer) => {
+const failure = (
+  bytes: Buffer,
+  reader:
+    | typeof readIl2cppMetadata
+    | typeof readIl2cppMetadataForLinking = readIl2cppMetadata,
+) => {
   try {
-    readIl2cppMetadata(bytes);
+    reader(bytes);
   } catch (cause: unknown) {
     if (cause instanceof Il2cppMetadataFormatFailure) return cause;
     throw cause;
@@ -161,7 +167,8 @@ describe("rejected IL2CPP global-metadata", () => {
       "unrecognized-magic",
       "unsupported",
     );
-    rejects(Buffer.alloc(4), "unrecognized-magic", "unsupported");
+    for (const length of [4, 5, 6, 7])
+      rejects(Buffer.alloc(length), "unrecognized-magic", "unsupported");
     rejects(
       mutated((_, bytes) => bytes.writeInt32LE(30, 4)),
       "unsupported-version",
@@ -169,6 +176,40 @@ describe("rejected IL2CPP global-metadata", () => {
     );
   });
 
+  it("retains the decoder's signed interpretation of a complete high-bit version", () => {
+    const bytes = Buffer.alloc(8);
+    bytes.writeUInt32LE(0xfab11baf, 0);
+    bytes.writeUInt32LE(0xffffffff, 4);
+    const rejected = failure(bytes);
+    expect(rejected.reason).toBe("unsupported-version");
+    expect(rejected.kind).toBe("unsupported");
+    expect(rejected.message).toContain("metadata version -1 is not supported");
+  });
+
+  it.each([4, 5, 6, 7])(
+    "retains recognized magic and reports a %i-byte version prefix as truncated",
+    (length) => {
+      const bytes = Buffer.alloc(length);
+      bytes.writeUInt32LE(0xfab11baf, 0);
+      const rejected = failure(bytes);
+      expect(rejected.reason).toBe("truncated-header");
+      expect(rejected.kind).toBe("corrupt");
+      expect(rejected.message).toContain(
+        "fewer than the 8-byte magic/version prefix",
+      );
+      expect(rejected.message).toContain("metadata version is unknown");
+    },
+  );
+
+  it("uses the same truncated-prefix classification before native linking", () => {
+    const bytes = Buffer.alloc(7);
+    bytes.writeUInt32LE(0xfab11baf, 0);
+    const rejected = failure(bytes, readIl2cppMetadataForLinking);
+    expect(rejected.reason).toBe("truncated-header");
+    expect(rejected.kind).toBe("corrupt");
+    expect(rejected.message).toContain("metadata version is unknown");
+  });
+
   it("rejects a truncated header and an unexpected header size", () => {
     rejects(
       createIl2cppMetadataFixture().bytes.subarray(0, 200),

tests/boundary/il2cpp/il2cppMetadataProvider.test.ts

@@ -352,6 +352,41 @@ it("retains the exact empty source observation without claiming a decoded invent
   });
 });
 
+it("retains the exact source when recognized magic has no complete version", async () => {
+  const root = await createTestTempDirectory("rea-il2cpp-short-prefix-");
+  const path = join(root, "global-metadata.dat");
+  const bytes = Buffer.alloc(7);
+  bytes.writeUInt32LE(0xfab11baf, 0);
+  await writeFile(path, bytes);
+  const result = await inspectMetadata(path);
+
+  expect(result.ok).toBe(false);
+  if (result.ok) throw new Error("Expected truncated metadata prefix");
+  expect(result.error).toMatchObject({
+    _tag: "AnalysisInputError",
+    issues: [
+      {
+        path: ["path"],
+        reason: "invalid_format",
+        message: expect.stringMatching(
+          /truncated-header:.*metadata version is unknown/u,
+        ),
+      },
+    ],
+  });
+  expect(result.error.partialObservation).toMatchObject({
+    operation: "observe_il2cpp_metadata_source",
+    normalized_result: {
+      artifact: {
+        path,
+        bytes: bytes.length,
+        sha256: createHash("sha256").update(bytes).digest("hex"),
+      },
+      inventory_status: "not-returned-as-complete",
+    },
+  });
+});
+
 it("keeps the metadata source observation when the selected binary does not link", async () => {
   const root = await createTestTempDirectory("rea-il2cpp-binary-failure-");
   const path = join(root, "global-metadata.dat");