DynamoDB JSON

DynamoDB attribute-value JSON serialization. The backend walks docast.Document.metadata (the StructuredMap channel) and emits the `{ "S": … } / { "N": … } / { "M": … } / { "L": … }` dialect — StructuredMap→M, StructuredList→L, StringScalar→S, IntegerScalar→N structurally, with the document-ast@1.3.0 esc-dynamodb-bool / -number / -null hints dispatching BOOL / N-from-string / NULL. A pure docast.Document → string serializer with no AWS dependency; throws if the Document has no metadata channel.