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>
Phase 1: Merge N+1 transfer() calls into a single transfer() per
Sync(PatchList), reducing N+1 MPI_Waitall barriers to 1 via new
Sync_merged() that collects all intra-patch and inter-patch grid
segment lists into combined per-rank arrays.
Phase 2: Cache grid segment lists and reuse grow-only communication
buffers across RK4 substeps via SyncCache struct. Caches are per-level
and per-variable-list (predictor/corrector), invalidated on regrid.
Eliminates redundant build_ghost_gsl/build_owned_gsl0/build_gstl
rebuilds and malloc/free cycles between regrids.
Phase 3: Split Sync into async Sync_start/Sync_finish to overlap
Cartesian ghost zone exchange (MPI_Isend/Irecv) with Shell patch
synchronization. Uses MPI tag 2 to avoid conflicts with SH->Synch()
which uses transfer() with tag 1.
Also updates makefile.inc paths and flags for local build environment.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
