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).

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.18.0"
+version = "0.18.1"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"