`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.
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