After the new assistant_turn lands, publish a `turn_html_replace` SSE
event carrying the rendered HTML, the new turn_id, and the original
assistant_turn id as `supersedes_id` so connected tabs can swap the
prior DOM node in-place. Phase 1 T29 deferred this — page had to refresh
to see the regenerated turn.
Uses a new event name (not the existing `turn_html`) because the HTMX
`sse-swap="turn_html"` consumer expects raw HTML and an *append*
semantic; regenerate is a *replace*. The new event ships as JSON
(supersedes_id forces sse.py's JSON branch) so the front-end JS can
read the swap target from the payload.
Test: `test_regenerate_broadcasts_turn_html_over_sse` patches the
`publish` reference inside the regenerate module and asserts the
event shape.
Joseph Doherty