ffi 2.6 xfail: #desc("(Sig)Ret") per-method JNI descriptor override

`weirdMethod :: (self: *Self) -> s32 #desc("()I");` should parse,
today's 2.5 parser expects `;` immediately after the return type
and errors at the `#desc` token. The make-green follow-up adds a
`hash_desc` lexer token and threads an optional `desc_override`
field through `JniMethodDecl`.

examples/ffi-jni-class-06-desc.sx

@@ -0,0 +1,20 @@
+// Phase 2 step 2.6 (PLAN-FFI.md): xfail then green for the
+// `#desc("(Sig)Ret")` per-method JNI-descriptor override.
+//
+// The default descriptor is derived from sx types (Phase 2.8); `#desc`
+// is the escape hatch when the auto-derived signature doesn't match
+// (e.g., synthetic methods, ambiguous overloads, accessing JNI
+// internals). It appears trailing the method's return type, before
+// the terminating `;`. Today the parser expects `;` immediately after
+// the return type and errors at the `#desc` token.
+
+#import "modules/std.sx";
+
+Foo :: #jni_class("com/example/Foo") {
+    weirdMethod :: (self: *Self) -> s32 #desc("()I");
+}
+
+main :: () -> s32 {
+    print("parse-only ok\n");
+    0;
+}

tests/expected/ffi-jni-class-06-desc.exit

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

tests/expected/ffi-jni-class-06-desc.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-class-06-desc.sx:14:41: error: expected ';'