Stage 2 of the faithful libyaml scanner port. Translates yaml_parser_save_simple_key, yaml_parser_remove_simple_key, yaml_parser_stale_simple_keys, and the flow-level push/pop (yaml_parser_increase_flow_level / decrease) onto a new PureYAML.Parsing.LibYAML.Scanner aggregate that embeds the indent engine. The simple-key mechanism is what lets a quoted scalar, a nested collection, or a property-carrying node become a mapping key when a ':' follows, and is required exactly at the block indent. Modelled faithfully: a potential key goes stale once it spans a line break or exceeds 1024 characters, and a required key that is removed or goes stale is the 'could not find expected colon' error. Unit-tested for the required/not-required, removal-error, staleness, and flow-level cases. No public behavior changed yet.