feat(platform): add FreeBSD support for interface statistics

Implement interface statistics for FreeBSD using sysctl and introduce a basic UCI configuration source for Linux. Include a cross-compilation configuration and a target check script to verify multi-platform support.

check.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+targets=(
+  aarch64-apple-darwin
+  x86_64-unknown-linux-gnu
+  x86_64-unknown-linux-musl
+  aarch64-unknown-linux-musl
+  x86_64-unknown-freebsd
+)
+
+for target in "${targets[@]}"; do
+  echo
+  echo "==> Checking ${target}"
+  cargo check \
+    --workspace \
+    --all-targets \
+    --target "${target}"
+done