Slice 4 of #94 (completes the epic). Quick-fixes are derived from the structured
schema completions, never by parsing a diagnostic message:
- TreeNode gains contentRange (the span between an element's start and end tags),
captured by the ranged reader, so an inserted attribute or child is placed
exactly.
- QuickFix and TextEdit model an editor edit (a replacement over a source range;
an insertion is a zero-width range).
- QuickFixEngine turns a Completions into fixes: a required-but-absent attribute
becomes an insertion before the start tag's >, and a still-expected required
child becomes an insertion before the end tag. Schema.Document.quickFixes(at:in:)
is the entry over a ranged tree.
Tests apply a fix and re-validate: adding the required id, or inserting the
required qty, yields XML the schema then accepts; a complete element offers none.
#94 (editor integration) is complete: ranged recovering read, unified lint,
schema completions, and quick-fixes.
