Port the located value tree onto the hand-rolled engine, reusing the
engine-agnostic located layer (Model.LocatedValue, LocatedPair,
SourcePosition, SourceRange, and the located composer helper) authored
on the conformance branch. The streaming composer now builds a parallel
located tree behind an opt-in tracksLocations flag, exposed as
parseLocated / parseStreamLocated; the plain Value path stays
byte-identical, gated behind the flag.
The scalar event carries the scanner token's end mark, so a located
scalar spans the whole token (end exclusive, one past the last
character) rather than being start-anchored. The hand-rolled scanner
already tracked token end marks, so this surfaces information the engine
was already computing.
Also stop emitting a spurious null document for a comment-only or empty
stream: YAML 1.2 gives such a stream zero documents, while the
single-document value API still maps it to null, so parse("") is
unchanged.
A whole-suite test proves the located projection equals a plain parse
for every passing case; the value, event-dump, and conformance suites
stay green (507 tests).
