ffi 2.4 xfail: #extends Alias; body item in #jni_class

`Window :: #jni_class("...") { #extends View; ... }` should parse,
today's 2.3 parser doesn't recognise `#extends` as a token and the
body loop reports "expected method name". The make-green follow-up
adds `hash_extends`/`hash_implements` lexer tokens, refactors
`JniClassDecl.methods` into a `members` tagged union, and dispatches
in the body loop on the leading token.

examples/ffi-jni-class-04-extends.sx

@@ -0,0 +1,25 @@
+// Phase 2 step 2.4 (PLAN-FFI.md): xfail then green for the `#extends`
+// and `#implements` body items inside a `#jni_class` declaration.
+//
+// `#extends Alias;` declares a single-inheritance superclass reference
+// using the sx-side alias name (not the foreign Java path — that lives
+// in the alias's own `#jni_class(...)` directive arg). `#implements
+// Alias;` is repeatable and records interface conformance.
+//
+// Step 2.4 introduces `hash_extends` and `hash_implements` tokens and
+// refactors `JniClassDecl.methods` into a `members` tagged union that
+// holds method/extends/implements variants.
+
+#import "modules/std.sx";
+
+View :: #jni_class("android/view/View") { }
+
+Window :: #jni_class("android/view/Window") {
+    #extends View;
+    getDecorView :: (self: *Self) -> *View;
+}
+
+main :: () -> s32 {
+    print("parse-only ok\n");
+    0;
+}

tests/expected/ffi-jni-class-04-extends.exit

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

tests/expected/ffi-jni-class-04-extends.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-class-04-extends.sx:18:5: error: expected method name in '#jni_class' body