feat(contracts): add MxSparseArray write-only value for default-fill partial writes

src/ZB.MOM.WW.MxGateway.Contracts/Protos/mxaccess_gateway.proto

@@ -1040,6 +1040,7 @@ message MxValue {
     google.protobuf.Timestamp timestamp_value = 16;
     MxArray array_value = 17;
     bytes raw_value = 18;
+    MxSparseArray sparse_array_value = 19;
   }
 }
 
@@ -1062,6 +1063,21 @@ message MxArray {
   }
 }
 
+// Write-only sparse array value. The gateway expands this into a full,
+// default-filled MxArray before forwarding to the worker; the worker never
+// receives or produces it. Unmentioned indices take the element type's
+// default (reset, NOT preserved).
+message MxSparseArray {
+  MxDataType element_data_type = 1;
+  uint32 total_length = 2;
+  repeated MxSparseElement elements = 3;
+}
+
+message MxSparseElement {
+  uint32 index = 1;
+  MxValue value = 2; // scalar
+}
+
 message BoolArray {
   repeated bool values = 1;
 }