Completes issue #10. Canonical mode follows libyaml's canonical layout
on both the Model and Tagged dump paths (tags explicit, scalars double
quoted, flow one entry per line, anchors and aliases preserved on the
Tagged path). explicitDocumentStart/End wrap the body in --- and ...
markers. lineWidth wraps overflowing flow collections one entry per
line and folds plain block scalars at single-space boundaries, vetoing
any fold that would manufacture syntax, so values always reparse
unchanged.
Closes#10
Closes two of the four emitter gaps a feature-parity audit against Yams
found (PureYAMLYamsComparison's FEATURE-PARITY.md): no configurable
indentation and no sorted-key output.
indentWidth replaces every hardcoded indent + 2 in Dumper.swift/
Dumper.Mapping.swift, verified with round-trip tests since indentation is
purely a rendering choice and must never change what a document means.
sortKeys sorts a mapping's pairs by their string key in both block and flow
style, but only when every key in that mapping is a plain string; a mapping
with any complex (mapping/sequence) key stays in insertion order, since
there is no well-defined ordering across mixed key kinds. Off by default,
since insertion order is what makes a duplicate-key-preserving Mapping
round-trip meaningfully in the first place.
width-based line wrapping and canonical mode are each a correctness surface
of their own and are deliberately left for later, not folded into this.
