fix(deploy): prevent 'Address already in use' from stale IPAM and half-torn-down containers

Two compounding root causes produced the recurring 'Address already in use'
error on redeploy:

1. _ensure_network only compared driver+name; if a prior deploy's IPAM
   pool drifted (different subnet/gateway/range), Docker kept handing out
   addresses from the old pool and raced the real LAN. Now also compares
   Subnet/Gateway/IPRange and rebuilds on drift.

2. A prior half-failed 'up' could leave containers still holding the IPs
   and ports the new run wants. Run 'compose down --remove-orphans' as a
   best-effort pre-up cleanup so IPAM starts from a clean state.

Also surface docker compose stderr to the structured log on failure so
the agent's journal captures Docker's actual message (which IP, which
port) instead of just the exit code.

tests/test_deployer.py

@@ -40,6 +40,7 @@ class TestCompose:
     @patch("decnet.engine.deployer.subprocess.run")
     def test_compose_constructs_correct_command(self, mock_run):
         from decnet.engine.deployer import _compose
+        mock_run.return_value = MagicMock(returncode=0, stdout="", stderr="")
         _compose("up", "-d", compose_file=Path("test.yml"))
         mock_run.assert_called_once()
         cmd = mock_run.call_args[0][0]
@@ -50,6 +51,7 @@ class TestCompose:
     @patch("decnet.engine.deployer.subprocess.run")
     def test_compose_passes_env(self, mock_run):
         from decnet.engine.deployer import _compose
+        mock_run.return_value = MagicMock(returncode=0, stdout="", stderr="")
         _compose("build", env={"DOCKER_BUILDKIT": "1"})
         _, kwargs = mock_run.call_args
         assert "DOCKER_BUILDKIT" in kwargs["env"]