ffi 2.16b xfail: omitted env in #jni_call inside #jni_env

`#jni_call(void)(target, "name", "sig")` (3 args before the first
string literal) should work inside an enclosing `#jni_env(env) { ... }`
scope, picking up the env from the block's value directly. Today's
lowering expects 4+ args and errors with "#jni_call requires env,
target, method name, and signature".

The make-green follow-up adds a lowering-side env stack maintained
across the `#jni_env` body walk, and a disambiguation in
`lowerJniCall` that detects "env omitted" via the position of the
first string-literal arg (method name at index 1 → omitted; at index
2 → explicit env).

examples/ffi-jni-env-02-lexical-direct.sx

@@ -0,0 +1,30 @@
+// Phase 2 step 2.16b (PLAN-FFI.md): xfail then green for the
+// lexical-direct env resolution inside a `#jni_env` scope.
+//
+// `#jni_call(T)(target, "name", "sig", args...)` — three args before
+// the first string literal — has the env omitted. The lowering walks
+// the in-progress AST visit stack to find the enclosing `#jni_env(env)`
+// block and uses that env value directly as the IR-level env arg.
+// No thread-local read, no per-call lookup; env stays register-
+// resident across loop bodies (the hot-path optimisation).
+//
+// Today's lower expects 4+ args and errors when called with 3.
+
+#import "modules/std.sx";
+
+g_should_call : bool = false;
+
+unused_jni :: (env: *void, target: *void) {
+    #jni_env(env) {
+        // Omitted env — env comes from the enclosing #jni_env scope.
+        #jni_call(void)(target, "noop", "()V");
+    }
+}
+
+main :: () -> s32 {
+    if g_should_call {
+        unused_jni(null, null);
+    }
+    print("ok\n");
+    0;
+}

tests/expected/ffi-jni-env-02-lexical-direct.exit

@@ -0,0 +1 @@
+1

tests/expected/ffi-jni-env-02-lexical-direct.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-env-02-lexical-direct.sx: error: #jni_call requires env, target, method name, and signature