One-line addition to the switch: `.f64 => Jni.CallDoubleMethod`.
First non-integer JNI return type; same lazy-cache + dispatch
infrastructure from 1.17 handles the rest.
IR snapshot updated: `ret double undef` replaced by the full
sequence through vtable slot 58 keyed on `("getValue", "()D")`.
Test-add for the jdouble return-type variant — `#jni_call(f64)(env,
target, "getValue", "()D")`. First non-integer return type for JNI.
IR snapshot captures today's `ret double undef` placeholder. The
next commit adds the `.f64 => Jni.CallDoubleMethod` arm.