feat: web SPA bind — add 'combined' (:8392) both-plane option as default

The bind dropdown offered only memory/affect single-plane binds; #18's
composite endpoint (:8392, both planes in one session) was never reachable
from the SPA. Add 'combined' as the default-selected option, keeping
memory-only / affect-only for single-plane isolation diagnostics.

- endpoint_for_plane: combined -> :8392 (sessions.py)
- web server: accept bifrost_plane="combined" (server.py)
- dropdown: combined (:8392) default-selected, single-plane retained (index.html)
- #17 contract: endpoint_for_plane FN + plane-selector spec updated to combined
- tests: endpoint_for_plane combined, server combined bind -> :8392, dropdown default

Suite 506 green. Live-verified on :8765 (current code).

tests/test_web_server.py

@@ -837,6 +837,50 @@ class TestWebBifrostBind:
         }
         assert upstream.headers["Authorization"] == "Bearer server-ck"
 
+    @respx.mock
+    def test_combined_plane_binds_to_8392(self) -> None:
+        """combined [#18 composite]: a 'combined' plane from the browser → the server
+        binds the :8392 both-plane endpoint; bound-state echoes plane='combined'."""
+        import json as _json
+
+        from ratatoskr.web.server import create_app
+
+        route = respx.post("https://w.example/sessions").mock(
+            return_value=httpx.Response(201, json=_CREATE_OK)
+        )
+        app = create_app(
+            _mock_client_factory(),
+            bifrost_consumer_key="server-ck",
+            bifrost_visible_host="10.100.10.50",
+        )
+        resp = TestClient(app).post(
+            "/api/sessions",
+            json={"agent_id": "ratatoskr:sindra", "bifrost_plane": "combined"},
+        )
+        assert resp.status_code == 201
+        assert resp.json()["bifrost"] == {
+            "plane": "combined",
+            "endpoint": "http://10.100.10.50:8392",
+            "status": "bound",
+        }
+        upstream = route.calls[0].request
+        body = _json.loads(upstream.content)
+        assert body["bifrost"] == {
+            "endpoint_url": "http://10.100.10.50:8392",
+            "scope": None,
+        }
+
+    def test_dropdown_offers_combined_as_default(self) -> None:
+        """(a)+default: the SPA plane dropdown offers a 'combined' (:8392) option,
+        it is the DEFAULT-selected one, and single-plane memory/affect remain."""
+        from pathlib import Path
+
+        import ratatoskr.web as web_pkg
+
+        html = (Path(web_pkg.__file__).parent / "static" / "index.html").read_text()
+        assert '<option value="combined" selected>' in html
+        assert 'value="memory"' in html and 'value="affect"' in html
+
     @respx.mock
     def test_plane_without_server_config_is_400(self) -> None:
         """A plane requested but no server-held key/host → bifrost_not_configured."""