Tagged.Scalar/Sequence/Mapping gain anchor and style fields
(Tagged.ScalarStyle, Tagged.CollectionStyle), Tagged.Node gains an
alias(anchor:node:) case carrying both the reference and its resolution,
and parseTagged populates all of it from the parse events instead of
discarding anchors and expanding aliases into copies.
PureYAML.dump(_ node: Tagged.Node, options:) re-emits anchors, alias
references, tags, and per-node styles over the document-wide policy.
Plain scalars re-emit their source spelling (NO stays NO, an empty null
stays empty); literal blocks cover keep chomping, single-line strip
chomping, leading blank lines, and whitespace-led first lines via an
explicit indentation indicator; folded scalars re-emit as > when the
folding rules invert exactly and degrade to a literal block otherwise.
The round-trip law parseTagged(dump(parseTagged(d))) == parseTagged(d)
is corpus-tested over every real-world fixture, with one documented
degradation: single-quoted content containing line breaks re-emits
double-quoted, value unchanged.
Closes#9
