Two coordinated changes:

1. sambaFull overlay extended to build with SMB-Direct support:
   - rdma-core added to buildInputs (provides libibverbs + librdmacm)
   - --with-smb-direct passed via configureFlags so waf wires up the
     transport layer at compile time

2. settings.global gains `smb direct = yes` + 8 MiB read/write knobs
   matching the NFS rsize/wsize on the same fabric. smbd now advertises
   capability 0x40 on protocol negotiate; clients that speak SMB-Direct
   (Win Server / Win Pro for Workstations / macOS Sequoia 15.4+) can
   upgrade SMB3 sessions onto the bond40g RoCE fabric. Clients without
   SMB-Direct silently fall back to plain TCP on 445.

The 2×40 GbE bond40g (ConnectX-3, post-cutover 2026-05-15) is the same
RDMA fabric NFS uses; SMB-Direct shares it without contention since
the queue-pair fanout is per-session. The "10 GbE NIC" comment in the
settings block is stale — replaced with the current 80 Gbps reality.

Build cost: sambaFull overlay forces a local rebuild on deploy
(~10-15 min, one CPU bound on smbd compilation).

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>