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.
