Add native Tofu (uTofu) gather-scatter backend

[njansson]

Adds a native Tofu (uTofu) one-sided RDMA gather-scatter backend for
Fugaku/Tofu-D, as a faster successor to the coarray (CAF) backend.

• Selected with NEKO_GS_COMM=UTOFU (or comm_bcknd=GS_COMM_UTOFU);
  built with --with-utofu[=DIR] (links -ltofucom, gated on HAVE_UTOFU).
• A send is a single utofu_put straight into the neighbour's receive
  buffer, with the slab tag fused into the put's edata — one network
  op per peer, no separate atomic/credit message.
• Per-instance send/recv buffers and a private receive VCQ/MRQ per
  gs_t; double-buffered send/receive with an unpack-as-ready receive
  loop. Packing is parallel over the OpenMP team; puts are issued by the
  master thread (concurrent puts across VCQs are unsupported on TofuD).
• NEKO_GS_UTOFU_NTNI selects the number of injection TNIs (default 1);
  NEKO_GS_UTOFU_CACHE_INJECT toggles Tofu cache injection.
• Docs (appendices, performance, installation, man page) and build deps
  updated.