feat(client-dotnet): add WriteArrayElementsAsync default-fill helper and document semantics

Adds a public WriteArrayElementsAsync helper on MxGatewaySession that builds
an MxValue{SparseArrayValue} and delegates to the existing WriteAsync. Extracts
the MxValue construction into an internal static BuildSparseArray builder for
unit-testability. Two new tests cover builder output shape and the full write
command path. README documents the reset (not preserve) semantics alongside
the existing whole-array guidance.

clients/dotnet/README.md

@@ -168,6 +168,30 @@ 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.
 
+When only a few indices need changing and the rest should be reset to the
+element type's default, use `WriteArrayElementsAsync` instead of building the
+full array manually:
+
+```csharp
+await session.WriteArrayElementsAsync(
+    serverHandle, itemHandle,
+    elementDataType: MxDataType.Integer,
+    totalLength: 20,
+    elements: new Dictionary<uint, MxValue>
+    {
+        [2] = 42.ToMxValue(),
+        [7] = 99.ToMxValue(),
+    });
+```
+
+The gateway expands the sparse descriptor into a full `totalLength`-element
+array before forwarding to the worker. Indices not listed in `elements` are
+written as the element type's default — this is a **reset**, not a preserve;
+current values at those positions are discarded. `totalLength` is required and
+must match the declared length of the array attribute. Bare-name array items
+(`Area001.Pump001.Speed`) are auto-normalized to the `[]` form at `AddItem`
+so the array attribute accepts the write.
+
 ## CLI Usage
 
 The test CLI supports deterministic JSON output for automation: