fix(mt): keep C declarations before statements

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

lib/compress/zstdmt_compress.c

@@ -2724,6 +2724,7 @@ static size_t ZSTDMT_createCompressionJob(void* opaque, size_t srcSize, unsigned
     ZSTDMT_CCtx* const mtctx = (ZSTDMT_CCtx*)opaque;
     ZSTD_EndDirective const endOp = (ZSTD_EndDirective)end;
     ZSTDMT_RustCreateJobProjection projection;
+    ZSTDMT_RustCreateJobResult result;
     /* Rust fills the reusable input range in its local projection before
      * calling this C-owned job-construction callback.  Publish that count
      * before the callback prepares a job and checks the C context. */
@@ -2743,7 +2744,7 @@ static size_t ZSTDMT_createCompressionJob(void* opaque, size_t srcSize, unsigned
         (unsigned)(endOp == ZSTD_e_end),
         (unsigned)mtctx->params.fParams.checksumFlag
     };
-    ZSTDMT_RustCreateJobResult const result = ZSTDMT_rust_createCompressionJob(
+    result = ZSTDMT_rust_createCompressionJob(
         &projection, mtctx, ZSTDMT_prepareCompressionJob,
         ZSTDMT_writeEmptyCompressionJob, ZSTDMT_tryAddCompressionJob);