Slice 4 of #92. The XSD complex-type and identity-constraint validators move
onto the validation framework, producing located results.
- ComplexValidator threads a coding path through its (inherently recursive)
descent, so every content-model, attribute, and simple-type violation is now
a ValidationError located by path ("... at path: order/qty"). Child steps
carry a sibling index only when a name repeats.
- IdentityValidator returns ValidationError instead of Issue.
- A new XSD rules enum wraps both as Validation<Node, XSDContext> values gated to
the document root, combined by Validator<XSDContext>. XSDContext carries the
type table, the identity constraints, and the root declaration.
- Schema.Document.validate builds the context and runs the validator, returning
[ValidationError].
XSD content-model validation is recursive by nature (a child's type depends on
its position), so the rules delegate to the recursion while contributing to one
error collection. Schematron is the last validator on Issue and migrates next.
Add ComplexValidator validating an element against a ComplexType: attribute
uses (required, typed, unknown-attribute rejection) and the content model
(empty, simpleContent, elementOnly, mixed). Particles compile to a Thompson
NFA over element names honoring minOccurs/maxOccurs with sequence and choice
compositors; all groups are validated order-independently by counting. Child
elements are validated recursively against their declared types.
Closes#9
