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/README.md

@@ -145,6 +145,18 @@ the unchanged elements included. For example, to change 2 elements of a
 the 2 new ones). Sending only the 2 changed values overwrites the attribute
 with a 2-element array.
 
+`Session.WriteArrayElements` offers a default-fill shorthand: pass only the
+indices you want to set along with a `totalLength`. The gateway expands the
+sparse representation into a full array before forwarding to MXAccess — every
+unmentioned index receives the element type's zero value (boolean `false`,
+integer `0`, float `0.0`, string `""`, time = Unix epoch). This is a **RESET**
+of unmentioned indices, not a preserve of existing values. Use the full-array
+form (read-modify-write) when existing element values must be preserved.
+
+`AddItem` (and `AddItem2`) now auto-normalize a bare attribute name to the `[]`
+array address form expected by MXAccess, so callers do not need to append `[]`
+themselves. Both forms are accepted; duplicates are deduplicated by the gateway.
+
 ## Galaxy Repository browse
 
 The `GalaxyRepository` service (proto package `galaxy_repository.v1`) is a