Port the relate-picker's relate-delete behaviour from Playwright into an SX
harness test that drives the real engine (web/engine.sx + web/orchestration.sx)
against the OCaml runner's in-memory mock DOM. Builds the candidate row, runs
process-elements to bind the form's submit, mocks fetch-request to return the
host's empty 200, fires submit, and asserts the row is deleted in place — the
full fetch→swap→DOM-mutation loop in pure SX.
Mock-DOM completeness (run_tests.ml): NodeList.item(i) so dom-query-all can
iterate querySelectorAll results, and a DOMParser mock so the empty-body
sx-swap=delete path (handle-html-response → parseFromString) works as in a
browser.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>