ffi 2.16a xfail: #jni_env(env) { body } block-form intrinsic

`#jni_env(synth_env) { ... }` should parse as a block-scoped env
intrinsic, today the lexer doesn't know the directive and the parser
errors at the `#` token in expression position. The make-green
follow-up adds the `hash_jni_env` lexer token, parser arm in
parsePrimary, AST node, and sema acceptance — body runs as a normal
block, env captured for later. TL push/pop semantics + optional env
in `#jni_call` land in 2.16b.

tests/expected/ffi-jni-env-01-block.exit

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

tests/expected/ffi-jni-env-01-block.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-env-01-block.sx:17:5: error: unexpected token in expression