Replace the notImplemented stub with a real parser built on a pull-based,
streaming event core that never requires the whole document in memory.
- Parsing.EventReader: pulls characters through a source closure with a bounded
lookahead buffer and emits one Parsing.Event at a time, holding only an
open-element stack. Drives chunked/streamed input.
- Parsing.Parser: builds a Model.Node tree iteratively over the event core
(explicit stack, no recursion).
- Reader: streaming character cursor (closure-backed, bounded buffer) replacing
the previous whole-string Array.
- EntityDecoder: the five predefined entities and decimal/hex char references.
- Handles elements, attributes, text, comments, CDATA, processing instructions,
and an XML declaration in the prolog. Rejects <!DOCTYPE> by default (no
DTD/XXE/entity-expansion exposure).
- Top-level API: parse(_:), parse(pulling:), events(_:), events(pulling:).
- 21 tests including event-stream and chunked-source coverage.
