feat(client-go): add WriteArrayElements default-fill helper and document semantics

Regenerate Go proto types from mxaccess_gateway.proto so MxSparseArray,
MxSparseElement, and MxValue_SparseArrayValue appear in the generated
package; add MxSparseArray/MxSparseElement type aliases to types.go;
add Session.WriteArrayElements and the unexported buildSparseArrayValue
builder; add three unit tests covering the sparse oneof structure,
empty-map case, and the round-trip through WriteArrayElements; update
README with default-fill reset semantics and auto-normalize note.

clients/go/mxgateway/session.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"sort"
 	"sync"
 	"time"
 
@@ -580,6 +581,57 @@ func (s *Session) WriteRaw(ctx context.Context, serverHandle, itemHandle int32,
 	})
 }
 
+// WriteArrayElements writes a sparse, default-filled array: only the given
+// elements (index → scalar value) are set; every unmentioned index up to
+// totalLength is written as the element type's default (false / 0 / "" / Unix
+// epoch for time). The gateway expands the sparse representation into a full
+// array write before forwarding to MXAccess — this is a RESET of unmentioned
+// indices, not a preserve. Neither RESET semantics nor the original array
+// content are retained.
+//
+// elementDataType must be a scalar MXAccess type (Boolean, Integer, Float,
+// Double, String, or Time). totalLength must be at least as large as the
+// highest index in elements plus one.
+func (s *Session) WriteArrayElements(
+	ctx context.Context,
+	serverHandle, itemHandle int32,
+	elementDataType MxDataType,
+	totalLength uint32,
+	elements map[uint32]*MxValue,
+	userID int32,
+) error {
+	return s.Write(ctx, serverHandle, itemHandle, buildSparseArrayValue(elementDataType, totalLength, elements), userID)
+}
+
+// buildSparseArrayValue constructs the MxValue carrying an MxSparseArray oneof
+// arm from a map of index → scalar MxValue. Keys are visited in ascending
+// order so the produced slice is deterministic (important for test assertions).
+func buildSparseArrayValue(elementDataType MxDataType, totalLength uint32, elements map[uint32]*MxValue) *MxValue {
+	indices := make([]uint32, 0, len(elements))
+	for idx := range elements {
+		indices = append(indices, idx)
+	}
+	sort.Slice(indices, func(i, j int) bool { return indices[i] < indices[j] })
+
+	sparseElements := make([]*MxSparseElement, 0, len(elements))
+	for _, idx := range indices {
+		sparseElements = append(sparseElements, &MxSparseElement{
+			Index: idx,
+			Value: elements[idx],
+		})
+	}
+
+	return &MxValue{
+		Kind: &pb.MxValue_SparseArrayValue{
+			SparseArrayValue: &MxSparseArray{
+				ElementDataType: elementDataType,
+				TotalLength:     totalLength,
+				Elements:        sparseElements,
+			},
+		},
+	}
+}
+
 // PingRaw sends a diagnostic PING command and returns the raw reply.
 // The message is echoed back by the gateway in the reply's DiagnosticMessage field.
 func (s *Session) PingRaw(ctx context.Context, message string) (*MxCommandReply, error) {