feat: regenerate broadcasts turn_html over SSE (T73.1)

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.

tests/test_regenerate.py

@@ -271,3 +271,85 @@ def test_regenerate_404_when_assistant_turn_missing(client, tmp_path):
         assert response.status_code == 404
     finally:
         app.dependency_overrides.clear()
+
+
+def test_regenerate_broadcasts_turn_html_over_sse(
+    tmp_path, monkeypatch
+):
+    """T73.1: regenerate publishes a ``turn_html_replace`` SSE event so
+    connected tabs swap the prior turn's DOM node in place.
+
+    The event carries:
+    - ``data``: rendered HTML for the new turn
+    - ``turn_id``: event_id of the new assistant_turn
+    - ``supersedes_id``: event_id of the original assistant_turn
+    """
+    import asyncio
+
+    from chat.config import Settings
+    from chat.db.migrate import apply_migrations
+    from chat.services import regenerate as regenerate_module
+    from chat.services.regenerate import regenerate_assistant_turn
+
+    db_path = tmp_path / "test.db"
+    cfg = tmp_path / "config.toml"
+    cfg.write_text('featherless_api_key = "test"\n')
+    monkeypatch.setenv("CHAT_CONFIG_PATH", str(cfg))
+    monkeypatch.setenv("CHAT_DB_PATH", str(db_path))
+    apply_migrations(db_path)
+
+    ut_id, at_id = _seed_with_one_turn(db_path)
+
+    published: list[tuple[str, dict]] = []
+
+    async def _capture(chat_id, event):
+        published.append((chat_id, event))
+
+    # Patch the imported reference inside the regenerate module so the
+    # service's call site goes through our spy.
+    monkeypatch.setattr(regenerate_module, "publish", _capture)
+
+    narrative_canned = "Refreshed reply."
+    state_canned = json.dumps(
+        {"affinity_delta": 0, "trust_delta": 0, "knowledge_facts": []}
+    )
+    canned = [narrative_canned, state_canned, state_canned]
+    mock_client = MockLLMClient(canned=list(canned))
+
+    settings = Settings(featherless_api_key="test")
+
+    with open_db(db_path) as conn:
+        new_text = asyncio.run(
+            regenerate_assistant_turn(
+                conn,
+                mock_client,
+                settings=settings,
+                chat_id="chat_bot_a",
+                original_assistant_event_id=at_id,
+            )
+        )
+        assert new_text == narrative_canned
+
+        # Find the new assistant_turn event_id for cross-checking.
+        cur = conn.execute(
+            "SELECT id FROM event_log "
+            "WHERE kind = 'assistant_turn' AND id != ? "
+            "AND superseded_by IS NULL",
+            (at_id,),
+        ).fetchone()
+        new_at_id = cur[0]
+
+    # Filter out per-token publishes; we want the replace broadcast.
+    replace_calls = [
+        ev for (_cid, ev) in published if ev.get("event") == "turn_html_replace"
+    ]
+    assert len(replace_calls) == 1
+    payload = replace_calls[0]
+    assert payload["supersedes_id"] == at_id
+    assert payload["turn_id"] == new_at_id
+    # The HTML carries the new narrative text and the speaker name.
+    assert "Refreshed reply." in payload["data"]
+    assert "BotA" in payload["data"]
+    # Sanity: every publish targeted this chat.
+    for cid, _ev in published:
+        assert cid == "chat_bot_a"