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
Joseph Doherty