A flow-sequence single-pair implicit key is single-line
(c-s-implicit-json-key uses s-separate-in-line), so its ':' may not cross
a line break: '[ "key"\n :value ]' is malformed. A flow MAPPING adjacent
value may still span lines ('{ "foo"\n :bar }' stays valid), which is why
the check is scoped to flow levels that are sequences.
Track per-flow-level whether it is a sequence ('[' vs '{') in a
flowIsSequence stack pushed/popped with the flow level, and in fetchValue
reject a pending simple key whose ':' crossed a line break when the
current flow level is a sequence.
Surpasses libyaml on ZXT5 (5MUD, the flow-mapping analogue, stays valid);
whole-suite conformance 397/402, canonical events 308/308.
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.
