fix: add explicit RDMA build constraints

Gate native RDMA, cuObject, and cuobjclient implementations behind the
rdma build tag while keeping fallback stubs available for standard builds.
Preserve the separate cuobjclient_host configuration, clarify unsupported
platform errors, and update Makefile RDMA targets to pass the required tags
and disable VCS stamping.

Co-authored-by: Ben McClelland <ben.mcclelland@versity.com>

cuobjclient/session_host_linux.go

@@ -12,7 +12,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//go:build linux && amd64 && cgo && cuobjclient_host
+//go:build linux && amd64 && cgo && rdma && cuobjclient_host
 
 // This file implements the cuObjClient Session API on top of the host-memory
 // RDMA client (package rdma/hostclient). It contains no CUDA/GPU dependency and

cuobjclient/session_linux.go

@@ -12,7 +12,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//go:build linux && amd64 && cgo && !cuobjclient_host
+//go:build linux && amd64 && cgo && rdma && !cuobjclient_host
 
 package cuobjclient
 

cuobjclient/session_stub.go

@@ -12,7 +12,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//go:build !(linux && amd64 && cgo)
+//go:build !(linux && amd64 && cgo && rdma)
 
 package cuobjclient
 
@@ -37,7 +37,7 @@ func NewSession(size int) (*Session, error) {
 	if size > MaxTransferSize {
 		return nil, fmt.Errorf("invalid size %d: exceeds MaxTransferSize (%d)", size, MaxTransferSize)
 	}
-	return nil, fmt.Errorf("cuobjclient: NewSession is only supported on linux/amd64 with cgo")
+	return nil, fmt.Errorf("cuobjclient: NewSession is only supported on linux/amd64 with cgo and the rdma build tag")
 }
 
 // Close is a no-op in the unsupported-platform stub.
@@ -52,7 +52,7 @@ func (s *Session) Upload(base *s3lib.Client, bucket, key string, src []byte) err
 	_ = bucket
 	_ = key
 	_ = src
-	return fmt.Errorf("cuobjclient: Upload is only supported on linux/amd64 with cgo")
+	return fmt.Errorf("cuobjclient: Upload is only supported on linux/amd64 with cgo and the rdma build tag")
 }
 
 // Download always returns an unsupported-platform error on this build.
@@ -62,5 +62,5 @@ func (s *Session) Download(base *s3lib.Client, bucket, key string, dst []byte) e
 	_ = bucket
 	_ = key
 	_ = dst
-	return fmt.Errorf("cuobjclient: Download is only supported on linux/amd64 with cgo")
+	return fmt.Errorf("cuobjclient: Download is only supported on linux/amd64 with cgo and the rdma build tag")
 }

cuobjclient/transport.go

@@ -12,7 +12,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//go:build linux && amd64 && cgo
+//go:build linux && amd64 && cgo && rdma
 
 // This file holds the S3 request plumbing shared by the GPU (session_linux.go)
 // and host-memory (session_host_linux.go) session implementations: both issue