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.

tests/test_paths.py

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Tests for the shared runtime-path probe."""
+from __future__ import annotations
+
+from decnet.paths import resolve_runtime_path
+
+
+def _resolve(tmp_path, env=None, runtime_dir=None):
+    return resolve_runtime_path(
+        "x.sock",
+        env_var="DECNET_TEST_PATH",
+        runtime_dir=str(runtime_dir if runtime_dir is not None else tmp_path),
+        user_fallback="~/.decnet/x.sock",
+    )
+
+
+def test_env_override_wins(tmp_path, monkeypatch):
+    monkeypatch.setenv("DECNET_TEST_PATH", "/explicit/here.sock")
+    assert _resolve(tmp_path) == "/explicit/here.sock"
+
+
+def test_writable_runtime_dir(tmp_path, monkeypatch):
+    monkeypatch.delenv("DECNET_TEST_PATH", raising=False)
+    assert _resolve(tmp_path) == str(tmp_path / "x.sock")
+
+
+def test_falls_back_when_runtime_dir_absent(tmp_path, monkeypatch):
+    monkeypatch.delenv("DECNET_TEST_PATH", raising=False)
+    missing = tmp_path / "nope"  # does not exist → not a writable dir
+    result = _resolve(tmp_path, runtime_dir=missing)
+    assert result.endswith("/.decnet/x.sock")
+    assert "~" not in result  # tilde expanded