Add HTML table output format (-O html)

[vmvarela]

Description

Add html to the output format list. Render SQL query results as an HTML <table> element.

sql-pipe data.csv -O html 'SELECT * FROM t LIMIT 5'
# <table>
# <thead><tr><th>name</th><th>age</th></tr></thead>
# <tbody>
# <tr><td>Alice</td><td>30</td></tr>
# <tr><td>Bob</td><td>25</td></tr>
# </tbody>
# </table>

Motivation

HTML tables are useful for embedding query results in web pages, email reports, and internal dashboards. The existing XML escaping infrastructure (writeXmlEscaped in xml.zig) can be reused for HTML entity escaping.

Acceptance Criteria

• Add html to the OutputFormat enum in src/format.zig
• Implement HTML table writer with <table>, <thead>, <tbody>, <tr>, <th>, <td> tags
• HTML-escape all cell values (reuse writeXmlEscaped pattern)
• NULL values render as empty cells
• Compatible with --header (controls <thead> vs <tbody> separation)
• Optional --html-class <class> flag for CSS class on the <table> element
• Integration tests cover the new format
• Help text updated

Implementation Notes

• Follow the same pattern as the XML output writer (format.zig lines 97-211)
• begin(): write <table>\n<thead><tr> + <th> for each column
• writeRow(): write <tr> + <td> for each value
• end(): write </tbody></table>\n
• About 40 lines of new code