One-line addition: `.bool => Jni.CallBooleanMethod`. The lazy-cache
+ dispatch from 1.17 handles the rest. JNI's `jboolean` is i8 in the
C ABI but always carries 0 or 1; LLVM's call boundary truncates the
return byte to i1 and the sx-level bool reads the low bit
canonically.
IR snapshot updated: `ret i1 undef` replaced by the full sequence
through vtable slot 37 keyed on `("isShown", "()Z")`.
Test-add for the jboolean return. JNI `jboolean` is a single byte (0
or 1); sx's `bool` lowers to LLVM `i1` with byte-coercion at the ABI
boundary. CallBooleanMethod is at vtable slot 37.
IR snapshot captures today's `ret i1 undef`. Next commit adds the
`.bool => Jni.CallBooleanMethod` arm.
