feat(uci): add generalized UCI configuration source

Introduce the UciSource abstraction as a config-rs Source that reads
UCI packages and maps them into the configuration tree. Supports
package selection, required/optional behavior, and custom config/save
directories. Move rust-uci dependency to cfg(unix) to enable UCI
support on all Unix systems for development and testing, while
Linux/OpenWrt remains the production-supported platform.

src/settings/mod.rs

@@ -9,6 +9,9 @@ use serde::{Deserialize, Deserializer, de};
 use std::fmt::{self, Display};
 use std::str::FromStr;
 
+#[cfg(all(feature = "uci", unix))]
+use crate::platform::unix::uci::UciSource;
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum MeasurementType {
     Icmp = 1,
@@ -76,6 +79,9 @@ impl Settings {
     pub(crate) fn load() -> Result<Self, ConfigError> {
         let builder = Config::builder();
 
+        #[cfg(all(feature = "uci", unix))]
+        let builder = builder.add_source(UciSource::new(["sqm-autorate-rust"]).required(false));
+
         #[cfg(feature = "toml")]
         let builder = builder.add_source(
             config::File::with_name("/etc/sqm-autorate-rust/config.toml").required(false),