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.

tests/topology/test_deploy.py

@@ -51,7 +51,7 @@ async def repo(tmp_path):
 
 @pytest.mark.anyio
 async def test_dry_run_writes_compose_and_preserves_pending(repo, tmp_path, monkeypatch):
-    monkeypatch.chdir(tmp_path)
+    monkeypatch.setenv("DECNET_RUN_DIR", str(tmp_path))
     plan = generate(_cfg())
     tid = await persist(repo, plan)
 
@@ -235,3 +235,22 @@ async def test_deploy_and_teardown_against_real_docker(repo, tmp_path, monkeypat
         p.unlink()
     # Sanity: Path roundtrip still resolvable
     assert isinstance(Path(str(p)), Path)
+
+
+def test_compose_path_is_absolute_and_cwd_independent(tmp_path, monkeypatch):
+    """Regression: a CWD-relative compose path littered the install dir and
+    let teardown's unlink() miss orphans. Path must be absolute and stable
+    across CWD changes so write and teardown always agree."""
+    monkeypatch.setenv("DECNET_RUN_DIR", str(tmp_path))
+    tid = "abcdef1234567890"
+
+    monkeypatch.chdir(tmp_path)
+    p1 = _topology_compose_path(tid)
+    sub = tmp_path / "elsewhere"
+    sub.mkdir()
+    monkeypatch.chdir(sub)
+    p2 = _topology_compose_path(tid)
+
+    assert p1.is_absolute()
+    assert p1 == p2, "compose path must not depend on process CWD"
+    assert p1.parent == tmp_path