dist: nccl ffi + comm bootstrap

New crate xtrain-distributed (mirrors xserv-distributed): hand-written NCCL
FFI (GetUniqueId / CommInitRank / AllReduce / CommDestroy / Group{Start,End},
ncclUniqueId passed by value per the NCCL ABI) and a safe DdpContext wrapper —
rank 0 mints the UniqueId, every rank inits its communicator under a group, and
all_reduce_average_grads in-place AllReduce(sum)s each param's .grad() device
buffer then scales by 1/world (reuses T7's scale_inplace kernel). AllReduce runs
on the null stream so it orders with the model's kernels (no extra barrier).

build.rs follows the per-crate convention: no nvcc -> no_cuda cfg (crate
compiles to empty, cargo check passes host-side); with nvcc, links -lnccl
-lcudart like xserv-distributed's build.rs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -7,6 +7,7 @@ members = [
     "crates/xtrain-model",
     "crates/xtrain-optim",
     "crates/xtrain-train",
+    "crates/xtrain-distributed",
 ]
 
 [workspace.package]