feat(van-weather): port weather service to Rust

Add a Rust implementation with privacy-masked locations, HTTP retries,
and Home Assistant sensor publishing. Package it with Nix and switch the
systemd service from Python to the compiled binary.

rust/van_weather/package.nix

@@ -0,0 +1,13 @@
+{ rustPlatform }:
+rustPlatform.buildRustPackage {
+  pname = "van-weather";
+  version = "0.1.0";
+  src = ./.;
+
+  cargoLock.lockFile = ./Cargo.lock;
+
+  meta = {
+    description = "Privacy-masked van weather publisher for Home Assistant";
+    mainProgram = "van-weather";
+  };
+}