fix(go): classify JVM class files instead of universal Mach-O for CAFEBABE bytes (#2458)

* test(go): pin JVM class files as unsupported Go images

The regression fails on main: readGoBinaryImage(javaClassFile()) reports
"Universal/fat Mach-O containers require selecting a thin slice" for a
JVM class file whose bytes 4..7 are minor/major version, not nfat_arch.

* fix(go): classify JVM class files instead of universal Mach-O

Route the four FAT magics through the shared readFatContainerHeader and
classifyFatContainerIdentity facts added for the same disambiguation in
the artifact and target lanes (921bb9575). Keep the thin-slice failure
only when the container identity is confirmed Mach-O; a JVM class file
now falls through to the generic only-ELF/PE/thin-Mach-O failure.

* test(go): pin universal Mach-O thin-slice failure with FAT fixtures

Cover the kept behavior with the repository's own fatImage/machoImage
builders (narrow and FAT64 tables) so the regression also proves real
universal containers keep the slice-selection failure.

* test(go): cover little-endian FAT magics in the universal container control

* fix(go): require positive universal Mach-O identity

Require positive Mach-O identity before reporting a universal thin-slice remedy; malformed FAT-shaped bytes are not proof. Add the JVM constant-pool CPU lookalike to the existing fixture control.

Source inspection and git diff --check; tests/builds/typechecks/CI reruns skipped at explicit user request.

* fix(go): retain malformed FAT header diagnostics

Preserve the owning FAT header parser reason for malformed headers while requiring positive Mach-O proof before the universal-slice remedy.

Source inspection and git diff --check only. Tests, builds and typechecks skipped by explicit user request. GitHub review writes paused globally.

---------

Co-authored-by: morluto <76467478+morluto@users.noreply.github.com>

tests/fixtures/javaClassFile.ts

@@ -1,5 +1,5 @@
 /** Build a valid minimal JVM class file, optionally with an unused UTF-8 constant. */
-export const javaClassFile = (extraUtf8Bytes = 0): Buffer => {
+export const javaClassFile = (extraUtf8Bytes = 0, fatCpuLookalike = false): Buffer => {
   const utf8 = (value: string): Buffer => {
     const bytes = Buffer.from(value, "utf8");
     const entry = Buffer.alloc(3 + bytes.length);
@@ -13,6 +13,7 @@ export const javaClassFile = (extraUtf8Bytes = 0): Buffer => {
     utf8("Hello"),
     Buffer.from([7, 0, 4]),
     utf8("java/lang/Object"),
+    ...(fatCpuLookalike ? [utf8("x"), Buffer.from([3, 0, 0, 0, 7])] : []),
     ...(extraUtf8Bytes === 0 ? [] : [utf8("x".repeat(extraUtf8Bytes))]),
   ];
   const headerAndPool = Buffer.alloc(10);