[Feature] Add HTML URL rewriting infrastructure

[Feature] Add HTML URL rewriting infrastructure

Implements infrastructure for rewriting clickable URLs in HTML content:

- Add span tracking to HTML parser to capture byte offsets of href/src attribute values
- Implement patch-based URL rewriting engine with overlap validation
- Add C→Lua glue for URL rewriting callback functions
- Support MIME re-encoding (quoted-printable, base64, 8bit) for modified content
- Add configuration options: enable_url_rewrite, url_rewrite_lua_func, url_rewrite_fold_limit

The feature allows Lua callbacks to transform URLs while preserving HTML structure
and MIME encoding. Integration with milter REPLBODY support enables message body
replacement.