Slice 1 of #94 (editor integration). The keystone: a located validation finding
can now be mapped back to a source range.
- TreeNode gains a sourceRange (start/end Mark); SourceRange is a half-open span.
- PureXML.readTree builds a mutable, parent-aware, best-effort tree whose every
node carries its span (an element from the < of its start tag to past its end
tag; leaves over the text they consumed). Never throws, recovers in place,
returns located diagnostics.
- TreeNode.node(at:) navigates a validation coding path to its node using the
same canonical convention the validators build paths with (a sibling index
only when a name repeats), and sourceRange(at:) returns the span. So
Document.validate's ValidationError.codingPath resolves to characters in the
document.
Tests prove the spans, the navigation, the end-to-end bridge (an XSD finding to
a source range), and that ranged reading of invalid input never crashes.
