Rebuild the XPath engine on the parent-aware tree. Add an XPath node
abstraction (tree, attribute, namespace) with document order, and navigate
all thirteen axes: child, descendant, parent, ancestor, following-sibling,
preceding-sibling, following, preceding, attribute, namespace, self,
descendant-or-self, ancestor-or-self. Support the ./../@/// abbreviations
and processing-instruction() node tests. Reverse axes number nearest-first
for positional predicates; results return de-duplicated in document order.
The attribute axis excludes xmlns declarations; the namespace axis surfaces
in-scope namespace nodes.
Closes#40
Add PureXML.XPath: a compiled location-path query engine reimplemented from the
XPath 1.0 specification; no upstream code copied.
- Compiler parses a path into steps; Evaluator walks the tree returning a
Selection node-set (node or attribute) in document order.
- Forward axes only (child, descendant //, self ., attribute @), since the value
model has no parent pointers. Node tests: name, *, text(), node(), comment().
Predicates: [n], [@a], [@a='v'], [child], [child='v'].
- Public surface: XPath.Query (init compiles, evaluate/elements/strings), the
Selection enum (element / stringValue), QueryError, and the one-shot
PureXML.xpath(_:over:). Upward/sibling axes and the full expression language
are explicitly out of scope.
- 12 XPath tests; 61 total.
