refactor(mt): pass compression job callback directly

Make ZSTDMT_createCompressionJob match the exact callback type consumed by
ZSTDMT_rust_compressStreamGeneric. The callback now performs the existing
opaque and end-directive casts itself, so the redundant C forwarding shim can
be removed without changing scheduling, assertions, logging, or ABI behavior.

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.
- No cargo, make, native, fuzz, or other heavy tests run per task scope.

lib/compress/zstdmt_compress.c

@@ -2712,8 +2712,10 @@ static int ZSTDMT_tryAddCompressionJob(void* opaque, unsigned jobID)
     return POOL_tryAdd(mtctx->factory, ZSTDMT_compressionJob, &mtctx->jobs[jobID]);
 }
 
-static size_t ZSTDMT_createCompressionJob(ZSTDMT_CCtx* mtctx, size_t srcSize, ZSTD_EndDirective endOp)
+static size_t ZSTDMT_createCompressionJob(void* opaque, size_t srcSize, unsigned end)
 {
+    ZSTDMT_CCtx* const mtctx = (ZSTDMT_CCtx*)opaque;
+    ZSTD_EndDirective const endOp = (ZSTD_EndDirective)end;
     ZSTDMT_RustCreateJobProjection const projection = {
         mtctx->doneJobID,
         mtctx->nextJobID,
@@ -2757,13 +2759,6 @@ static size_t ZSTDMT_createCompressionJob(ZSTDMT_CCtx* mtctx, size_t srcSize, ZS
     return result.returnCode;
 }
 
-static size_t ZSTDMT_streamCreateJob(void* opaque, size_t srcSize, unsigned end)
-{
-    return ZSTDMT_createCompressionJob((ZSTDMT_CCtx*)opaque, srcSize,
-                                       (ZSTD_EndDirective)end);
-}
-
-
 /* The Rust flush state machine receives only this synchronized scalar view.
  * The descriptor, condition variable, serial checksum state, and buffer pool
  * remain private to C. */
@@ -3097,7 +3092,7 @@ size_t ZSTDMT_compressStream_generic(ZSTDMT_CCtx* mtctx,
         ZSTDMT_rust_compressStreamGeneric(
             &context, &inputProjection, &outputProjection, (unsigned)endOp,
             mtctx, ZSTDMT_streamTryGetInputRange, ZSTDMT_streamLoadInput,
-            ZSTDMT_streamCreateJob, ZSTDMT_streamFlushProduced);
+            ZSTDMT_createCompressionJob, ZSTDMT_streamFlushProduced);
 
     DEBUGLOG(5, "ZSTDMT_compressStream_generic (endOp=%u, srcSize=%u)",
                 (U32)endOp, (U32)(input->size - input->pos));