feat(config): add config-rs settings support

Introduce config-rs and Serde support with a nested Settings model.
Add config and serde dependencies with toml feature for file-based
configuration loading. The Settings struct provides typed configuration
with sensible defaults for network, output, observability, and advanced
settings sections. Environment variable override via SQMA__ prefix is
supported. The legacy Config remains in use; migration follows later.

src/main.rs

@@ -17,6 +17,7 @@ mod pinger_icmp_ts;
 mod platform;
 mod ratecontroller;
 mod reflector_selector;
+mod settings;
 mod time;
 mod util;