Broaden the serializer and writer options from the audit:
- attributeQuote selects single or double quotes; the active quote is
escaped in values (apostrophe as ' in single-quote mode).
- lineEnding controls the inserted newline (set to CRLF for \r\n output).
- xml:space=preserve suppresses pretty-print indentation for an element
and its subtree; xml:space=default re-enables it.
- Writer gains writeStartElementNS(prefix:localName:namespaceURI:) and
writeNamespace(prefix:uri:), so callers no longer hand-format prefixes
and xmlns attributes.
CDATA-vs-escape and entity re-escaping strategies are left as a
follow-up; the current escaping is the safe default.
Closes#91
