XML input and output format support

[vmvarela]

Part of #68
Depends on #95 (format plugin architecture)

Description

Add row-based XML as an input and output format. Each row maps to an XML element; column values become child elements or attributes.

Acceptance Criteria

• --input-format xml reads row-based XML (each child element of root = one row; grandchildren = columns)
• --output-format xml writes results as XML with configurable root/row element names
• --xml-root <name> flag sets the root element name (default: results)
• --xml-row <name> flag sets the row element name (default: row)
• Column values with special XML characters are properly escaped
• Malformed XML produces a clear error with line/column number
• Nested XML is handled: nested elements are serialized as XML strings

Notes

• Use Zig's std.xml or a minimal C XML library (expat is small and permissive)
• Row-based XML is simpler than columnar XML — target row-based only for now
• Example output:

  <results>
    <row><name>Alice</name><age>30</age></row>
    <row><name>Bob</name><age>25</age></row>
  </results>