ffi 1.20: #jni_call(f64) → CallDoubleMethod (slot 58) — make-green

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")`.

src/ir/emit_llvm.zig

@@ -1243,6 +1243,7 @@ pub const LLVMEmitter = struct {
                     .void => Jni.CallVoidMethod,
                     .s32 => Jni.CallIntMethod,
                     .s64 => Jni.CallLongMethod,
+                    .f64 => Jni.CallDoubleMethod,
                     else => {
                         self.mapRef(c.LLVMGetUndef(self.toLLVMType(instruction.ty)));
                         return;