`Lowering` gains a `jni_env_stack: ArrayList(Ref)`. When lowering
the `jni_env_block` arm pushes the env_expr's Ref before lowering
the body and pops after; `defer` ensures cleanup on early return.
`lowerJniCall` now disambiguates explicit-vs-omitted env via the
position of the first string-literal arg: at index 1 → omitted
(3-arg form `target, "name", "sig"`), at index 2 → explicit
(4-arg form `env, target, "name", "sig"`). Omitted form reads the
top of `jni_env_stack`; missing scope → diagnostic.
End-to-end test runs cleanly. Locked-in IR snapshot at
`tests/expected/ffi-jni-env-02-lexical-direct.ir` shows env coming
from the enclosing fn's `*void` param straight into the jni_msg_send
expansion — no extra load, no thread-local read. The hot-path
optimisation from the design discussion is now real.
128/128 examples + 1 new IR snapshot green; zig test clean.
`#jni_call(void)(target, "name", "sig")` (3 args before the first
string literal) should work inside an enclosing `#jni_env(env) { ... }`
scope, picking up the env from the block's value directly. Today's
lowering expects 4+ args and errors with "#jni_call requires env,
target, method name, and signature".
The make-green follow-up adds a lowering-side env stack maintained
across the `#jni_env` body walk, and a disambiguation in
`lowerJniCall` that detects "env omitted" via the position of the
first string-literal arg (method name at index 1 → omitted; at index
2 → explicit env).
