Rule-canon audit (validation-rules.md / the OpenAPIKit idiom): the streaming
XSD validator was the one surface applying a bare method (validateShallow)
rather than a Validation<> value. Wrap it: ComplexValidator.shallowValidity is
a Validation<ResolvedElement, ComplexValidator> whose subject carries the
element with its already-resolved effective type (the contextual typing a
node-tree walk cannot express), and the streaming driver applies it via
apply(to:at:in:). Now every validation surface, DTD, XSD, RELAX NG, Schematron,
HTML, structural, conformance, and both streaming validators, routes through a
Validation value. Adds a direct unit test of the rule.
Add Schema.Document.validate(streaming:) and ComplexValidator.validateShallow:
resolve each element's type from its parent's content model (and xsi:type /
typeReference) as it opens, then check its attributes and content-model
structure shallowly as it closes, against a shallow element synthesized from
the streamed child names. A child's deeper validity is checked when it closes,
so only the open-element stack is held. A differential test asserts streaming
agrees with the tree validator across valid and invalid documents (missing
attributes, bad simple values, missing/extra/misordered children). Wildcard
processContents and substitution-group resolution in the streaming form stay
on the tree validator. The streaming methods moved to ComplexValidatorStreaming
to keep ComplexValidator under the file/type-body caps.
Closes#107
