Add `parse_bind_addr`, `default_state_dir(subdir)`, and an `InvalidBindAddr` variant to `maestro-config`, and route the daemon, registry, and web config loaders through them. Each crate's hand-rolled `InvalidBindAddr` variant and duplicated XDG state-dir resolution are gone; their loaders now `?` the shared helper, which converts into their existing transparent `Inner` variant. Also adds the shared `BroadcastTuning` struct (used by the next commit). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>