fix(decompress): match Rust trace context ABI

Define the decompression trace projection with the exact unsigned-long-long
representation used by the public trace API.  The generic U64 typedef can be
a different C type with the same width, which made the Rust trace view fail to
compile when assigned the DCtx trace field.

Test Plan:
- ulimit -v 41943040; make -j1

lib/decompress/zstd_decompress.c

@@ -106,7 +106,10 @@ typedef struct {
     size_t dctx_size;
 } ZSTD_rustDctxView;
 
-typedef U64 ZSTD_rustTraceCtx;
+/* Rust exposes this as u64.  Use the trace API's exact unsigned-long-long
+ * representation instead of U64, whose typedef may be a different C type on
+ * some targets even when it has the same width. */
+typedef unsigned long long ZSTD_rustTraceCtx;
 typedef ZSTD_rustTraceCtx (*ZSTD_rustTraceBeginFn)(const void* dctx);
 typedef void (*ZSTD_rustTraceEndFn)(ZSTD_rustTraceCtx traceCtx,
                                     const void* trace);