Add parseLocated / parseStreamLocated, returning a Model.LocatedValue
tree parallel in shape to Value, where every node carries its source
span (1-based line/column) and located children. Collections span from
their opening to closing event; scalars carry a start-anchored span;
aliases and merge-inherited pairs honestly point at their reference site.
The plain Value, its equality, and the conformance oracle are untouched:
locations are built only when the streaming composer is asked for them
(tracksLocations), behind new entry points, so the default value path is
byte-identical. A whole-suite test proves the located projection equals
a plain parse for every passing case (402/402), including through merge
keys and aliases, and the canonical-event and gap gates stay green.
