docs: document MxSparseArray default-fill writes and bare-name array AddItem

docs/WorkerConversion.md

@@ -75,6 +75,20 @@ private static MxValue CreateNullValue(
 }
 ```
 
+### Sparse array expansion (write path, gateway only)
+
+`MxSparseArray` — the `sparse_array_value` arm on `MxValue` — is a write-only
+shorthand. The worker never produces or receives it; the gateway expands it into
+a full `MxArray` before the command reaches the named pipe. Expansion allocates
+a complete array of `total_length` slots, initializes every slot to the element
+type's default (bool → `false`; numeric → `0`; string → `""`; time/timestamp →
+Unix epoch), then writes each `MxSparseElement` at its declared index. The
+resulting `array_value` is an ordinary `MxArray` that passes through the
+conversion layer unchanged. The worker therefore still performs a single
+whole-array COM write, preserving MXAccess parity. Unmentioned indices are
+**reset** to their type default, not preserved from prior state — there is no
+read-modify-write merge.
+
 ### Array projection
 
 `ConvertArray` records the rank and per-dimension lengths so multi-dimensional `SAFEARRAY` shapes survive the round trip. The element type is resolved from the caller-supplied hint or the CLR element type via `ResolveArrayElementDataType`, then dispatched to the matching typed builder (`ConvertBoolArray`, `ConvertInt64Array`, `ConvertTimestampArray`, and so on).