MazeNET phase 2 step 2. Mirrors the flat-fleet service_config pattern
(DeckyConfig.service_config → composer → svc.compose_fragment) into the
topology compose pipeline, so a hand-authored decky can carry overrides
like {"ssh": {"password": "megapassword"}} and the ssh fragment reads
them just like the flat path does.
- _PlannedDecky gains service_config: dict[str, dict].
- persist() stores it under decky_config["service_config"].
- topology/compose.py passes cfg.get("service_config", {}).get(svc, {})
to svc.compose_fragment(service_cfg=...).
Schema unchanged — service_config lives inside the existing
decky_config JSON blob. Zero changes in decnet/services/*.
