feat(core): implement StaticConfig/DynamicConfig split with ArcSwap hot-reload

Split alknet-core configuration into StaticConfig (immutable after startup)
and DynamicConfig (hot-reloadable at runtime via ArcSwap).

- Add StaticConfig struct in config/static_config.rs with all fields per ADR-030
- Add DynamicConfig struct with AuthPolicy, ForwardingPolicy, RateLimitConfig
- Add ForwardingPolicy with allow_all()/deny_all() defaults (ADR-031)
- Add ConfigReloadHandle with reload() method for runtime config updates
- Replace Arc<ServerAuthConfig> with Arc<ArcSwap<DynamicConfig>> in ServerHandler
- Add config_reload_handle() to Server for obtaining reload handles
- Add AuthPolicy with authenticate_publickey/authenticate_certificate methods
- All existing tests pass with the new config structure
- Default DynamicConfig produces identical behavior to current code

crates/alknet-core/src/config/mod.rs

@@ -0,0 +1,8 @@
+pub mod dynamic_config;
+pub mod static_config;
+
+pub use dynamic_config::{
+    new_dynamic_config, AuthPolicy, ConfigReloadHandle, DynamicConfig, ForwardingAction,
+    ForwardingPolicy, ForwardingRule, RateLimitConfig,
+};
+pub use static_config::StaticConfig;