fix(engine): per-scope docker compose project names

Every compose invocation used -p decnet so fleet + every topology
lived in one docker compose project. --remove-orphans, run during
fleet pre-up cleanup and on every topology teardown / rollback, then
swept every container in the project not listed in the current compose
file — wiping sibling topologies and the flat fleet along with the
intended target.

Parameterize project on _compose / _compose_with_retry / _compose_ps
(default FLEET_COMPOSE_PROJECT="decnet"). Add _topology_compose_project
that returns decnet-topo-<id8>, and pass it through every topology
compose call site (master deploy_topology + rollback + post-deploy ps,
master teardown_topology, agent apply, agent teardown, all four live
service mutations on topology deckies). Fleet calls keep the default
and are unaffected.

Migration: live containers from before this fix remain in the shared
"decnet" project and need a one-time manual cleanup before they're
reachable to the new topology code paths.

tests/engine/test_services_live.py

@@ -75,7 +75,7 @@ async def test_topology_add_service_persists_and_runs_compose_up(
 ) -> None:
     captured: list[tuple[str, ...]] = []
 
-    def fake_compose(*args, compose_file=None, env=None):
+    def fake_compose(*args, compose_file=None, env=None, project=None):
         captured.append(args)
 
     monkeypatch.setattr(services_live, "_compose", fake_compose)