fix(il2cpp): validate generic-container extents before index widths

Validate the fixed 16-byte version-39 records before using their count to select the serialized index width. Reject contradictory count and byte extents while preserving valid zero, one and 300-record controls.

Validation: 89 focused reader/provider/compiled CLI and stdio MCP tests; check:ci; seven reproduced CLI and SDK-validated MCP cases with unchanged fixture hashes. Version-specific source and independent parser evidence distinguish fixed records from compressed indices. Synthetic metadata only; no real-game or target-execution qualification.

Fixes #5751

tests/boundary/il2cpp/il2cppMetadataFile.test.ts

@@ -261,13 +261,6 @@ describe("rejected IL2CPP global-metadata", () => {
       ),
       "record-size-mismatch",
     );
-    // 300 generic containers widen GenericContainerIndex to two bytes.
-    rejects(
-      mutated((fixture, bytes) =>
-        bytes.writeInt32LE(300, fixture.header("genericContainers", "count")),
-      ),
-      "record-size-mismatch",
-    );
     rejects(
       mutated((fixture, bytes) =>
         bytes.writeInt32LE(2, fixture.header("nestedTypes", "count")),
@@ -290,6 +283,62 @@ describe("rejected IL2CPP global-metadata", () => {
   });
 });
 
+describe("IL2CPP generic-container extents", () => {
+  it("checks generic-container count against its fixed 16-byte records before resolving widths", () => {
+    // A positive count with no record bytes.
+    for (const count of [1, 300])
+      rejects(
+        mutated((fixture, bytes) =>
+          bytes.writeInt32LE(
+            count,
+            fixture.header("genericContainers", "count"),
+          ),
+        ),
+        "record-size-mismatch",
+      );
+    const oneRecord = createIl2cppMetadataFixture({
+      wide: { fillerTypes: 0, genericContainers: 1, parameters: 0 },
+    });
+    // A positive count with a partial record.
+    rejects(
+      mutated(
+        (fixture, bytes) =>
+          bytes.writeInt32LE(1, fixture.header("genericContainers", "size")),
+        oneRecord,
+      ),
+      "record-size-mismatch",
+    );
+    // Bytes without a corresponding record.
+    rejects(
+      mutated(
+        (fixture, bytes) =>
+          bytes.writeInt32LE(0, fixture.header("genericContainers", "count")),
+        oneRecord,
+      ),
+      "record-size-mismatch",
+    );
+  });
+
+  it("retains width selection for empty and complete generic-container records", () => {
+    const empty = readIl2cppMetadata(createIl2cppMetadataFixture().bytes);
+    expect(empty.index_widths.generic_container).toBe(1);
+
+    const one = readIl2cppMetadata(
+      createIl2cppMetadataFixture({
+        wide: { fillerTypes: 0, genericContainers: 1, parameters: 0 },
+      }).bytes,
+    );
+    expect(one.index_widths.generic_container).toBe(1);
+
+    const wide = readIl2cppMetadata(
+      createIl2cppMetadataFixture({
+        wide: { fillerTypes: 0, genericContainers: 300, parameters: 0 },
+      }).bytes,
+    );
+    expect(wide.index_widths.generic_container).toBe(2);
+  });
+});
+
 describe("rejected IL2CPP metadata references", () => {
   it("rejects image, type, method and nested-type indices outside their tables", () => {
     rejects(