`#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.
