New web/request-handler.sx: configurable SX function (sx-handle-request)
that receives path + headers + env and returns rendered HTML.
The handler decides full page vs AJAX fragment.
OCaml server: http_render_page now just calls the SX handler.
All routing, layout selection, AJAX detection moved to SX.
Header parsing added. is_sx_request removed from OCaml.
Configurable via SX_REQUEST_HANDLER env var (default: sx-handle-request).
WIP: handler has parse errors on some URL formats. Needs debugging.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
