Stage 3 foundation of the faithful libyaml scanner port. Translates libyaml's input buffer and position tracking (the buffer/mark fields and the PEEK/SKIP/SKIP_LINE macros) into PureYAML.Parsing.LibYAML.Reader. It iterates Unicode scalars rather than grapheme clusters so a CR and an LF are distinct, matching libyaml's byte-oriented scanning (a CR+LF pair is still one line break), and tracks the mark in libyaml's 0-based line and column convention. Unit-tested for peek/skip column tracking and LF and CRLF line breaks. This is the input layer the token fetchers consume.
