The MT job callback added state publication before its Rust job-construction
call, which left the result declaration after executable statements and emitted
the repository's C90 mixed-declaration warning on every CLI/test build.
Declare the result alongside the other callback-local projections and assign it
at the existing call site. This is a warning-only cleanup with no runtime or
ABI change.
Test Plan:
- `cc -fsyntax-only -Werror=incompatible-pointer-types -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -Ilib/legacy lib/compress/zstdmt_compress.c` -- passed
- `git diff --check` and `git diff --cached --check` -- passed
- The prior capped suite reached `rustLibSmoke`; a fresh root library build is
required because its archive predates the display-hook fix
ddidderr
66b04980eb