build(uci): restrict UCI dependency and feature to Linux targets

Ensure the UCI integration is only compiled on Linux, as the underlying native dependency is platform-specific. Added a build script to warn users when the feature is enabled on unsupported platforms.

build.rs

@@ -0,0 +1,16 @@
+use std::env;
+
+fn main() {
+    let uci_enabled = env::var_os("CARGO_FEATURE_UCI").is_some();
+    let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap_or_default();
+    let target = env::var("TARGET").unwrap_or_else(|_| "unknown".into());
+
+    if uci_enabled && target_os != "linux" {
+        println!(
+            "cargo::warning=feature `uci` is unsupported for target `{target}`; \
+             UCI integration will not be compiled"
+        );
+    }
+
+    println!("cargo::rerun-if-changed=build.rs");
+}