Add mppdisttool: unified MPP distribution tool

[underwoo]

Summary

Adds mppdisttool, a single pure-C binary with a subcommand interface that replaces seven separate MPP distribution programs:

Legacy tool	Subcommand
mppncscatter	scatter
mppnccombine	combine --mpp
combine-ncc	combine --land
scatter-ncc	scatter --land
decompress-ncc	decompress
iceberg_comb.sh	combine --iceberg / check --iceberg
combine_restarts	auto

The new binary eliminates the Fortran and NCO (ncrcat, ncatted) runtime dependencies, making these tools usable on HPC login, data-transfer, and service nodes that frequently lack the full Fortran module stack or NCO.

Subcommands

• combine — auto-detects file type (compressed-by-gathering → land path; iceberg restart → iceberg path; otherwise MPP path); mode can be forced with --land, --iceberg, or --mpp
• scatter — MPP domain decomposition or --land CF compressed scatter
• decompress — expand compressed-by-gathering files to full grid
• check — --compressed (replaces is-compressed) or --iceberg; exit codes preserved
• auto — scans CWD for *{res,nc}.#### files and routes each group through the appropriate combine path (replaces combine_restarts)

Backward compatibility

install-exec-hook installs symlinks for all six legacy binary names. argv[0] detection maps each symlink name to the correct subcommand and flags at startup, so existing scripts require no changes.

Source layout

src/mpp-disttool/
├── main.c               argv[0] compat + subcommand dispatch table
├── cmd_combine.c/h      MPP (mppnccombine, modernised nc_* API),
│                        land (combine-ncc.F90 port),
│                        iceberg (no ncrcat/ncatted)
├── cmd_scatter.c/h      MPP (mppncscatter/domain.c),
│                        land (scatter-ncc.F90 port)
├── cmd_decompress.c/h   decompress-ncc.F90 port
├── cmd_check.c/h        is-compressed + iceberg check (no ncdump)
├── cmd_auto.c/h         combine_restarts port (POSIX opendir + regcomp)
├── compress.c/h         nfu_compress.F90 port + rank_ascending merge-sort
├── nc_utils.c/h         nfu.F90 helpers (clone_dim/var, format detect)
├── domain.c/h           mpp_compute_extent, hyperslabcopy, scatter_dims
├── strlist.c/h          copied from mpp-ncscatter
└── xmalloc.c/h          copied from mpp-ncscatter

Sources are placed in mpp-disttool/ (not mppdisttool/) to avoid automake creating a same-named object subdirectory that shadows the binary target under make -j.

Build system changes

• src/Makefile.am — new mppdisttool target; install-exec-hook for six compatibility symlinks
• configure.ac — adds AC_PROG_LN_S so $(LN_S) is defined for the install hook

Test plan

• autoreconf -i && configure && make -j builds cleanly
• make install installs mppdisttool and all six symlinks
• Each symlink dispatches to the correct subcommand (--help output spot-check)
• Existing test suite passes: make check for mppncscatter, mppnccombine, combine-ncc, scatter-ncc, decompress-ncc, iceberg_comb, is-compressed, combine_restarts
• ldd mppdisttool confirms no Fortran runtime or NCO library dependency

Closes #405 (parallel build work tracked separately on nfu.parallel.build)

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