distributed: NCCL P2P primitives (PpContext + send/recv)

Add ncclSend/ncclRecv FFI and a PpContext that initializes a NCCL
communicator across P pipeline stages and hands the hidden state to
neighbour stages on the null stream. Mirrors TpContext; the collective
differs (point-to-point hand-off vs in-layer AllReduce).

tests/sendrecv.rs: 2-GPU stage0->stage1 send/recv smoke test.

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

crates/xserv-distributed/src/ffi.rs

@@ -45,6 +45,23 @@ unsafe extern "C" {
         comm: NcclComm,
         stream: CudaStream,
     ) -> i32;
+    // Point-to-point primitives for pipeline parallelism (Phase 18).
+    pub fn ncclSend(
+        sendbuff: *const c_void,
+        count: usize,
+        datatype: i32,
+        peer: i32,
+        comm: NcclComm,
+        stream: CudaStream,
+    ) -> i32;
+    pub fn ncclRecv(
+        recvbuff: *mut c_void,
+        count: usize,
+        datatype: i32,
+        peer: i32,
+        comm: NcclComm,
+        stream: CudaStream,
+    ) -> i32;
     pub fn ncclGroupStart() -> i32;
     pub fn ncclGroupEnd() -> i32;
     pub fn ncclGetErrorString(result: i32) -> *const c_char;