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/platform.rs

@@ -42,6 +42,9 @@ mod linux;
 #[cfg(target_os = "macos")]
 mod macos;
 
+#[cfg(unix)]
+pub mod unix;
+
 #[cfg(not(any(target_os = "freebsd", target_os = "linux", target_os = "macos")))]
 mod unsupported;