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/Cargo.toml

@@ -34,6 +34,7 @@ iroh = { version = "0.34", optional = true }
 url = { version = "2", optional = true }
 async-trait = "0.1"
 ipnetwork = "0.21.1"
+arc-swap = "1"
 
 [dev-dependencies]
 alknet-core = { path = ".", features = ["testutil", "tls", "iroh"] }