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/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "van-weather"
+version = "0.1.0"
+edition = "2024"
+description = "Fetch privacy-masked weather for a van and publish it to Home Assistant"
+license = "MIT"
+
+[dependencies]
+anyhow = "1.0"
+chrono = "0.4"
+clap = { version = "4.5", features = ["derive", "env"] }
+env_logger = "0.11"
+log = "0.4"
+reqwest = { version = "0.12", default-features = false, features = ["blocking", "json", "rustls-tls"] }
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+
+[lints.clippy]
+all = "deny"
+pedantic = "deny"
+