fix(topology): anchor compose path to run dir, stop install-dir litter

_topology_compose_path returned a CWD-relative Path, so every
deploy/mutate/dry-run wrote decnet-topology-<id8>-compose.yml into the
process CWD (the install dir). Teardown computed the same relative path
against its own CWD, so when it differed the unlink() missed the orphan
and files accumulated forever.

Anchor to $DECNET_RUN_DIR (default /var/lib/decnet/topologies, tempdir
fallback) so write and teardown always agree regardless of CWD.

decnet/engine/deployer.py

@@ -5,8 +5,10 @@ Deploy, teardown, and status via Docker SDK + subprocess docker compose.
 
 import asyncio
 import json
+import os
 import shutil
 import subprocess  # nosec B404
+import tempfile
 import time
 from pathlib import Path
 from typing import cast
@@ -833,7 +835,16 @@ def _teardown_order(lans: list[dict]) -> list[str]:
 
 
 def _topology_compose_path(topology_id: str) -> Path:
-    return Path(f"decnet-topology-{topology_id[:8]}-compose.yml")
+    # Anchor to a stable absolute dir so write and teardown agree
+    # regardless of process CWD — a CWD-relative path littered the
+    # install tree and let teardown's unlink() miss orphaned files.
+    base = Path(os.environ.get("DECNET_RUN_DIR", "/var/lib/decnet/topologies"))
+    try:
+        base.mkdir(parents=True, exist_ok=True)
+    except OSError:
+        base = Path(tempfile.gettempdir()) / "decnet-topologies"
+        base.mkdir(parents=True, exist_ok=True)
+    return base / f"decnet-topology-{topology_id[:8]}-compose.yml"
 
 
 def _topology_compose_project(topology_id: str) -> str: