distributed: NCCL tensor-parallel primitives (TpContext + AllReduce)

New xserv-distributed crate: hand-written NCCL FFI, TpContext (one rank per
thread, bound to one GPU), and in-place BF16 AllReduce on the null stream so
it orders naturally with the model's kernels. 2-GPU AllReduce test included.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/xserv-distributed/src/ffi.rs

@@ -0,0 +1,65 @@
+//! Minimal NCCL FFI bindings (hand-written, like the CUDA bindings).
+//! Only the collectives we need for tensor parallelism.
+
+use std::ffi::c_void;
+use std::os::raw::c_char;
+use xserv_cuda::ffi::CudaStream;
+
+/// Opaque NCCL communicator handle (`ncclComm_t`).
+pub type NcclComm = *mut c_void;
+
+/// `ncclUniqueId` is a 128-byte opaque blob shared from rank 0 to all ranks.
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub struct NcclUniqueId {
+    pub internal: [c_char; 128],
+}
+
+impl Default for NcclUniqueId {
+    fn default() -> Self {
+        Self { internal: [0; 128] }
+    }
+}
+
+// ncclDataType_t (subset)
+pub const NCCL_FLOAT32: i32 = 7;
+pub const NCCL_BF16: i32 = 9;
+
+// ncclRedOp_t
+pub const NCCL_SUM: i32 = 0;
+
+// ncclResult_t
+pub const NCCL_SUCCESS: i32 = 0;
+
+unsafe extern "C" {
+    pub fn ncclGetUniqueId(uid: *mut NcclUniqueId) -> i32;
+    // ncclUniqueId is passed BY VALUE (a 128-byte struct) per the NCCL ABI.
+    pub fn ncclCommInitRank(comm: *mut NcclComm, nranks: i32, commid: NcclUniqueId, rank: i32) -> i32;
+    pub fn ncclCommDestroy(comm: NcclComm) -> i32;
+    pub fn ncclAllReduce(
+        sendbuff: *const c_void,
+        recvbuff: *mut c_void,
+        count: usize,
+        datatype: i32,
+        op: i32,
+        comm: NcclComm,
+        stream: CudaStream,
+    ) -> i32;
+    pub fn ncclGroupStart() -> i32;
+    pub fn ncclGroupEnd() -> i32;
+    pub fn ncclGetErrorString(result: i32) -> *const c_char;
+}
+
+pub fn err_string(result: i32) -> String {
+    unsafe {
+        let p = ncclGetErrorString(result);
+        if p.is_null() {
+            return format!("nccl error {result}");
+        }
+        std::ffi::CStr::from_ptr(p).to_string_lossy().into_owned()
+    }
+}
+
+pub fn check(result: i32, what: &str) {
+    assert_eq!(result, NCCL_SUCCESS, "{what} failed: {}", err_string(result));
+}