Add YAML configuration model and loader

Config is layered: built-in defaults, then the YAML file, then BFD_* environment variables, then CLI flags. Global defaults live under a 'defaults' mapping; per-peer settings are file-only. Includes config.yaml.dist and ScriptHookSink::with_script_dir so the hooks directory is configurable.

src/lib.rs

@@ -1,4 +1,5 @@
 pub mod bfd;
+pub mod config;
 pub mod control;
 pub mod events;