feat(rust): add shared Signal client and workspace configuration

rust/van_weather/package.nix

@@ -1,10 +1,26 @@
-{ rustPlatform }:
+{ lib, rustPlatform }:
 rustPlatform.buildRustPackage {
   pname = "van-weather";
   version = "0.1.0";
-  src = ./.;
+  src = lib.fileset.toSource {
+    root = ../..;
+    fileset = lib.fileset.unions [
+      ../../Cargo.toml
+      ../../Cargo.lock
+      ../../clippy.toml
+      ../.
+    ];
+  };
 
-  cargoLock.lockFile = ./Cargo.lock;
+  cargoLock.lockFile = ../../Cargo.lock;
+  cargoBuildFlags = [
+    "-p"
+    "van-weather"
+  ];
+  cargoTestFlags = [
+    "-p"
+    "van-weather"
+  ];
 
   meta = {
     description = "Privacy-masked van weather publisher for Home Assistant";