Regenerate Python proto bindings to pick up MxSparseArray/MxSparseElement/
sparse_array_value from the shared mxaccess_gateway.proto. Add
Session.write_array_elements which builds an MxValue(sparse_array_value=…)
from a {index→scalar} dict and delegates to the existing write(). Add 8 pytest
tests covering builder correctness and full round-trip wire shape. Update
README with a default-fill semantics paragraph and bare-name array-write note.
Joseph Doherty