style(mt): keep serial reset C90-clean

The MT serial-reset bridge now declares its Rust scalar projection before
executing the diagnostic branch. This keeps the declaration ordering valid for
the C90-oriented compiler flags used by the native test builds while leaving
the Rust-owned policy and callback sequence unchanged.

Test Plan:
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 make -j1` -- passed without the
  new mixed-declaration warning.
- `git diff --cached --check` -- passed.

lib/compress/zstdmt_compress.c

@@ -1526,12 +1526,6 @@ ZSTDMT_serialState_reset(SerialState* serialState,
                          ZSTD_dictContentType_e dictContentType)
 {
     ZSTDMT_serialResetContext context;
-
-    /* Rust owns LDM normalization; retain the original diagnostic only. */
-    if (params.ldmParams.enableLdm == ZSTD_ps_enable) {
-        DEBUGLOG(4, "LDM window size = %u KB", (1U << params.cParams.windowLog) >> 10);
-    }
-
     ZSTDMT_RustSerialResetProjection const projection = {
         params.ldmParams.enableLdm,
         params.fParams.checksumFlag,
@@ -1547,6 +1541,11 @@ ZSTDMT_serialState_reset(SerialState* serialState,
         sizeof(ldmEntry_t)
     };
 
+    /* Rust owns LDM normalization; retain the original diagnostic only. */
+    if (params.ldmParams.enableLdm == ZSTD_ps_enable) {
+        DEBUGLOG(4, "LDM window size = %u KB", (1U << params.cParams.windowLog) >> 10);
+    }
+
     context.serialState = serialState;
     context.seqPool = seqPool;
     context.params = &params;