Adds _materialise_decky_{spawn,remove,connect,disconnect,services_diff,recreate_base}
helpers alongside the existing _materialise_lan_change. Each follows
the same skip rules: bail when topology is not active/degraded, when
agent-pinned, or when docker calls fail (logged, not re-raised — DB
remains source of truth).
apply_add_decky now calls _materialise_decky_spawn after the DB writes.
The helper:
* re-renders the per-topology compose so it lists the new decky;
* runs 'compose up -d --no-deps --build <decky_base> <decky>-<svc>...'
in a worker thread (matches engine/services_live's pattern).
Service container targets are filtered through get_service() so
fleet_singleton services are skipped — they don't have per-decky
compose entries. Gateway (forwards_l3=True) deckies need no
special-case here; the compose generator already emits the host
'ports:' block for them.
Subsequent commits wire the other apply_* ops to the matching
helpers. Tests for the full set ship in the workstream's last
commit.
anti
8c06190e69