Per ns-plain-first a '-' starts a plain scalar in flow only when a
flow-safe character follows. A '-' before a flow indicator ('[-]',
'[-, -]') cannot start a node, but the port (like libyaml) read it as the
scalar '-'.
Reject that form in the indicator dispatch. Extract scan_to_next_token,
the comment validator, and blankLineAhead into LibYAML.Scanner.SkipToToken
to keep the fetch file within the length limit.
Surpasses libyaml on G5U8, YJV2; whole-suite conformance 390/402,
canonical events 308/308.
