refactor: consolidate writable-dir probe into decnet/paths.py

bus.factory and vectorstore.factory carried byte-identical copies of the
'env override -> writable runtime dir -> ~/.decnet fallback' probe. Move
it to decnet.paths.resolve_runtime_path and call it from both.

The mkdir-create variants (deployer topologies dir, _pid_dir candidate
iteration, personas_pool existence-precedence) are deliberately left
inline: they're different policies, not the same probe.

decnet/paths.py

@@ -0,0 +1,41 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Shared runtime filesystem path resolution.
+
+DECNET writes runtime state under a system dir provisioned by ``decnet
+init`` / systemd (``/var/lib/decnet`` for state, ``/run/decnet`` for
+sockets). On dev boxes without systemd, or in CI, that dir may be absent
+or read-only, so callers fall back to a per-user location.
+
+:func:`resolve_runtime_path` centralises the writable-dir probe that the
+vectorstore and bus backends previously copy-pasted verbatim.
+"""
+from __future__ import annotations
+
+import os
+
+
+def resolve_runtime_path(
+    filename: str,
+    *,
+    env_var: str,
+    runtime_dir: str,
+    user_fallback: str,
+) -> str:
+    """Resolve a runtime file path. Creates nothing.
+
+    Precedence:
+      1. ``$env_var`` if set (used verbatim).
+      2. ``runtime_dir/filename`` if ``runtime_dir`` exists and is writable.
+      3. ``user_fallback`` (``~`` expanded).
+
+    ``runtime_dir`` is *probed*, never created: it is meant to be
+    provisioned with the right ownership and perms by init/systemd, so
+    creating it here with whatever perms the current process happens to
+    have would be worse than falling back to the user path.
+    """
+    explicit = os.environ.get(env_var)
+    if explicit:
+        return explicit
+    if os.path.isdir(runtime_dir) and os.access(runtime_dir, os.W_OK):
+        return os.path.join(runtime_dir, filename)
+    return os.path.expanduser(user_fallback)