ffi 1.18: #jni_call(s32) → CallIntMethod (slot 49) — make-green

One-line addition to the `call_method_offset` switch in
`emit_llvm.zig` — `.s32 => 49` (CallIntMethod). The 1.17 caching
infrastructure handles the rest: GetObjectClass → NewGlobalRef →
GetMethodID populate the shared `@SX_JNI_{CLS,MID}_<key>` pair on
miss; per-call lowering loads the cached jmethodID and dispatches
through vtable slot 49 with an `i32` return.

IR snapshot at `tests/expected/ffi-jni-call-04-jint-return.ir`
updated: the `ret i32 undef` placeholder is replaced by the full
lazy-cache + CallIntMethod sequence keyed on
`("getCount", "()I")`. Pre-1.18 snapshot was 1d7ea72.

src/ir/emit_llvm.zig

@@ -1220,6 +1220,7 @@ pub const LLVMEmitter = struct {
                 const ret_ty_id = instruction.ty;
                 const call_method_offset: u32 = switch (ret_ty_id) {
                     .void => 61, // CallVoidMethod
+                    .s32 => 49, // CallIntMethod
                     else => {
                         self.mapRef(c.LLVMGetUndef(self.toLLVMType(instruction.ty)));
                         return;