ffi 2.3 xfail: static method body item in #jni_class

`Math :: #jni_class("java/lang/Math") { static abs :: (n: s32) -> s32; }`
should parse, today's 2.2 parser treats `static` as a plain
identifier and errors at the following `abs`. The make-green
follow-up adds a `static` keyword recognition step in the body
loop and an `is_static` flag on `JniMethodDecl`.

examples/ffi-jni-class-03-static.sx

@@ -0,0 +1,19 @@
+// Phase 2 step 2.3 (PLAN-FFI.md): xfail then green for the `static`
+// method body item inside a `#jni_class` declaration.
+//
+// `static name :: (args...) -> Ret;` declares a class/static method —
+// no implicit `self`, dispatched via `GetStaticMethodID` / `CallStatic*`
+// at lowering time (Phase 2.12). Step 2.3 extends `parseJniClassDecl`
+// to recognise the `static` prefix and mark the resulting
+// `JniMethodDecl` with `is_static = true`.
+
+#import "modules/std.sx";
+
+Math :: #jni_class("java/lang/Math") {
+    static abs :: (n: s32) -> s32;
+}
+
+main :: () -> s32 {
+    print("parse-only ok\n");
+    0;
+}

tests/expected/ffi-jni-class-03-static.exit

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

tests/expected/ffi-jni-class-03-static.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-class-03-static.sx:13:12: error: expected '::'