Cache data_packer lengths in Sync_start to skip redundant buffer-size traversals

The data_packer(NULL, ...) calls that compute send/recv buffer lengths
traverse all grid segments × variables × nprocs on every Sync_start
invocation, even though lengths never change once the cache is built.
Add a lengths_valid flag to SyncCache so these length computations are
done once and reused on subsequent calls (4× per RK4 step).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AMSS_NCKU_source/Parallel.h

@@ -97,6 +97,7 @@ namespace Parallel
     MPI_Request *reqs;
     MPI_Status *stats;
     int max_reqs;
+    bool lengths_valid;
     SyncCache();
     void invalidate();
     void destroy();