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")`.
Test-add for the jlong return-type variant — same shape as 1.18's
jint test but exercising `#jni_call(s64)(env, target,
"currentTimeMillis", "()J")`. Today the non-void switch falls
through to `LLVMGetUndef`; the IR snapshot captures the placeholder.
The next commit adds the `.s64 => Jni.CallLongMethod` arm. The
snapshot will update to show the full dispatch through vtable slot
52, reusing the 1.17 slot interning machinery.
