fix: make --xml-root and --xml-row work for XML input parsing

[vmvarela]

Summary

• --xml-root and --xml-row were silently ignored when parsing XML input — they only controlled output element names
• The parser always used the document's actual root and accepted any child as a row, breaking nested structures like RSS feeds
• This fix adds navigation (navigateToRoot) and row filtering (nextRow filter) to the XML parser, and threads the flags through all input code paths

Changes

src/xml.zig

• XmlParser.skipElementBody() — new helper that skips a complete element tree (content + closing tag), handling nesting, CDATA, comments
• XmlParser.navigateToRoot(xml_root) — if the actual document root matches xml_root, proceeds directly; otherwise scans direct children of the actual root until the target is found, skipping non-matching siblings
• XmlParser.nextRow(..., row_tag_filter) — new optional param; wraps the read loop so non-matching elements are skipped via skipElementBody
• loadXmlInput, getXmlColumnNames, summarizeXml — all accept xml_root: ?[]const u8 and xml_row: ?[]const u8 (null = legacy behaviour)

src/main.zig

• ColumnsArgs, ValidateArgs, ParsedArgs — each gains xml_root_input: ?[]const u8 and xml_row_input: ?[]const u8
• When --xml-root/--xml-row are explicitly provided they flow to all three XML input functions; defaults leave both as null (no change in behaviour)

build.zig

• test_xml_no_rows: updated to use <results></results> (consistent with default xml_root = "results")
• Added integration test 114: nested navigation with --xml-root/--xml-row on an RSS-like document
• Added integration test 115: --validate with --xml-root/--xml-row counts only matching rows

Verification

zig build test   # all tests pass
ziglint src build.zig   # no warnings

Live test with the originally failing command:

curl -s "https://feeds.feedburner.com/TheHackersNews" \
  | sql-pipe -I xml --xml-root channel --xml-row item \
    'SELECT pubDate as Fecha, title as Noticia
     FROM t
     WHERE title LIKE "%Google%" OR title LIKE "%Apple%"
     ORDER BY Fecha DESC LIMIT 5'

Closes #139