fix(mt): preserve range state across job callback

The first MT state-publication fix made the C count current before job
construction, but the Rust result still unconditionally copied its local
pre-job fill count back into `mtctx` after the callback returned.  Preparing a
job clears `mtctx->inBuff.buffer` and `mtctx->inBuff.filled`, so the next stream
pass then observed a consumed range as if it were still active and returned a
generic error in the multi-file CLI test.

Keep the C-cleared state whenever job preparation has detached the active
range.  Rust's filled count is published only while the C range remains active,
which covers the blocked/table-full path where no job reset occurred.  The
callback still receives the current count before preparation, and the C90
projection declaration is kept before executable statements.

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 capped upstream suite reproduced a generic error in the multi-file CLI
  path before this correction; the full rerun is pending

lib/compress/zstdmt_compress.c

@@ -2723,12 +2723,13 @@ 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;
     /* 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. */
     assert(srcSize <= mtctx->inBuff.buffer.capacity);
     mtctx->inBuff.filled = srcSize;
-    ZSTDMT_RustCreateJobProjection const projection = {
+    projection = (ZSTDMT_RustCreateJobProjection){
         mtctx->doneJobID,
         mtctx->nextJobID,
         mtctx->jobIDMask,
@@ -3099,7 +3100,11 @@ size_t ZSTDMT_compressStream_generic(ZSTDMT_CCtx* mtctx,
                 (U32)endOp, (U32)(input->size - input->pos));
     assert(output->pos <= output->size);
     assert(input->pos  <= input->size);
-    mtctx->inBuff.filled = result.inBuffFilled;
+    /* A prepared job clears the C range while Rust still has the pre-job
+     * count in its local projection.  Only publish the result when the range
+     * remains active, which is the no-job-created path. */
+    if (mtctx->inBuff.buffer.start != NULL)
+        mtctx->inBuff.filled = result.inBuffFilled;
     input->pos = result.inputPos;
     output->pos = result.outputPos;
     DEBUGLOG(5, "end of ZSTDMT_compressStream_generic: remainingToFlush = %u", (U32)result.result);