ffi 2.1: parser accepts Foo :: #jni_class("path") { } opaque form

New `hash_jni_class` token + lexer entry, `JniClassDecl` AST node
(alias + java path; body deferred to 2.2+), `parseJniClassDecl`
consuming `("...") { }` and rejecting non-empty bodies for now.
Sema registers the alias as a type_alias symbol; LSP classifies
the directive as a keyword. The 2.0 xfail snapshot flips to
`parse-only ok`, exit 0.

120/120 examples green; zig test clean.

src/token.zig

@@ -115,6 +115,7 @@ pub const Tag = enum {
     hash_objc_call, // #objc_call(T)(recv, "sel:", args...)
     hash_jni_call, // #jni_call(T)(env, target, "name", "(Sig)R", args...)
     hash_jni_static_call, // #jni_static_call(T)(class, "name", "(Sig)R", args...)
+    hash_jni_class, // Foo :: #jni_class("java/path/Foo") { ...body... }
     triple_minus, // ---
 
     // Special