New `hash_jni_env` lexer token; `parsePrimary` dispatches to a small
`parseJniEnvBlock` that consumes `(env) { body }` and returns a new
`JniEnvBlock` AST node (env_expr + body block).
Sema's analyzeNode arm recurses into env + body inside a pushed
scope; findNodeAtOffset descends through both children for go-to-
definition.
Lowering treats it as a syntactic wrapper around the block: env is
evaluated for side effects, body lowers as a normal block. The TL
push/pop semantics (synthesizing the env stack so `#jni_call`'s env
arg can become optional) land in 2.16b.
`expectSemicolonAfter` recognises `jni_env_block` as block-form so
statement-position uses don't need a trailing `;` — matches `if` /
`while` / `for` / bare blocks.
Test runs through the block body and prints expected output; xfail
snapshot flips to green. 127/127 examples green.
`#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.
