Add PureXML.parse(pullingBytes:) and events(pullingBytes:) over an incremental
byte source (() -> UInt8?). StreamingDecoder detects the encoding from the
leading bytes, strips any BOM, and yields one scalar at a time as a Character,
so a byte stream is never fully buffered. UTF-8 (1 to 4 bytes) and UTF-16 (with
surrogate pairs) are decoded incrementally; invalid sequences become the Unicode
replacement character. Standard library only, no Foundation. Reimplemented from
the documented UTF decoding rules; no upstream code copied.
