build: thread-test package opts back into threading

Wave C dropped the .threaded flag the old addThreadedUserBinary call
carried, so thread-test compiled single_threaded: the heap mutex was
compiled out and atomics lowered to plain ops while the fixture spawns
real kernel threads — the QEMU thread-alloc case caught it (corruption
under concurrent allocation), and the completion review confirmed the
same finding independently. All 12 QEMU thread cases pass again.

test/system/services/thread-test/build.zig

@@ -11,6 +11,7 @@ pub fn build(b: *std.Build) void {
         .name = "thread-test",
         .root_source_file = b.path("thread-test.zig"),
         .domains = build_support.domains(b),
+        .threaded = true, // real atomics/TLS (docs/threading.md)
     });
     b.installArtifact(exe);
 }