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.

tests/expected/ffi-jni-class-01-empty.exit

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

tests/expected/ffi-jni-class-01-empty.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-jni-class-01-empty.sx:13:8: error: unexpected token in expression
+parse-only ok