feat(client-rust): add write_array_elements default-fill helper and document semantics

Handles the new MxSparseArray wire type (proto field 19 on MxValue::Kind):
- value.rs: map SparseArrayValue to MxValueProjection::Unset (write-only; never emitted on read path)
- session.rs: add write_array_elements() that builds the sparse proto value and delegates to write()
- tests: three unit tests asserting proto shape, empty-elements case, and read-path Unset projection
- README: document write_array_elements default-fill semantics and bare-name [] normalisation

clients/rust/README.md

@@ -168,6 +168,39 @@ 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.
 
+#### Default-fill partial array writes
+
+When you only need to set a handful of indices and want every other position to
+take the element type's default (zero / `false` / empty string / Unix epoch for
+timestamps), use `Session::write_array_elements` instead:
+
+```rust
+// Write a 10-element integer array; index 0 = 42, index 7 = 99,
+// all other indices default to 0 (not preserved from the previous value).
+session
+    .write_array_elements(
+        server_handle,
+        item_handle,
+        MxDataType::Integer,
+        10,
+        [(0, MxValue::int32(42)), (7, MxValue::int32(99))],
+        user_id,
+    )
+    .await?;
+```
+
+The gateway expands the sparse representation into a full `MxArray` before
+forwarding to the worker — the worker and MXAccess COM never see the sparse
+form. Unmentioned indices are reset to the type default, **not** preserved from
+the existing attribute value.
+
+#### Bare-name array AddItem normalisation
+
+`AddItem` for a bare array attribute name (e.g. `Tank01.Temperature`) is
+automatically normalised to `Tank01.Temperature[]` by the gateway so the
+worker can resolve the full array. You do not need to append `[]` in client
+code; the gateway handles it.
+
 ## Galaxy Repository browse
 
 The Galaxy Repository service exposes a read-only browse over the AVEVA System