feat(mutator,web): add_decky op — create-and-attach in one mutation

apply_attach_decky requires an existing decky, so the MazeNET editor
had no way to grow a live topology: creating a new decky on active
topologies 409'd on the direct-CRUD createDecky call.

- Backend: new apply_add_decky that creates the decky row + its
  home-LAN edge atomically, auto-allocating an IP if none pinned.
  Post-apply validation still runs. Added to DISPATCH + _MUTATION_OPS
  Literal + CLI help text.
- Tests: 3 new ops tests (happy path, duplicate-name rejection,
  missing-LAN rejection) plus dispatch coverage update.
- Frontend: useTopologyEditor gains addDeckyToLan() composite. Pending
  routes through createDecky + attachEdge as before; active routes
  through a single add_decky enqueue. MazeNET.tsx drag-archetype,
  duplicate, DMZ-gateway, and ctx-menu add-decky paths all use the
  composite so active topologies stop 409'ing on new-decky drops.

decnet/cli/topology.py

@@ -267,8 +267,8 @@ def _mutate(
     op: str = typer.Argument(
         ...,
         help=(
-            "One of: add_lan, remove_lan, attach_decky, detach_decky, "
-            "remove_decky, update_decky, update_lan"
+            "One of: add_lan, remove_lan, add_decky, attach_decky, "
+            "detach_decky, remove_decky, update_decky, update_lan"
         ),
     ),
     payload_json: str = typer.Option(