One-line addition to the `call_method_offset` switch: `.s64 =>
Jni.CallLongMethod`. The 1.17 caching infrastructure and the named-
constants struct from c1877fc handle the rest.
IR snapshot at `tests/expected/ffi-jni-call-05-jlong-return.ir`
updated: `ret i64 undef` replaced by the full lazy-cache +
CallLongMethod (vtable slot 52) sequence keyed on
`("currentTimeMillis", "()J")`.