mdtablefix converts <table>…</table> blocks that span multiple lines and
carry leading indentation into Markdown pipe tables. The leading indentation is
preserved on every emitted row, so the converted table sits at the same
indentation level as the original HTML. Surrounding non-table lines at that
same indentation level are passed through unchanged. Nested <table> tags are
tracked by depth, so the buffered structure is converted only once the outermost
</table> is reached and never split into two separate conversions.