Make Windows PATH install idempotent

tests/release_scripts.rs

@@ -145,6 +145,20 @@ fn windows_installer_reuses_persistent_source_update_cache() {
     );
 }
 
+#[test]
+fn windows_installer_adds_user_path_idempotently() {
+    let ps1 = include_str!("../install.ps1");
+    assert!(ps1.contains("function Add-UserPath($PathToAdd)"));
+    assert!(ps1.contains("$wanted = Normalize-PathForCompare $PathToAdd"));
+    assert!(ps1.contains("-ieq $wanted"));
+    assert!(ps1.contains("(@($entries) + $PathToAdd) -join ';'"));
+    assert!(
+        !ps1.contains("\"$userPath;$bindir\""),
+        "Windows installer must not write a leading semicolon when user PATH is empty"
+    );
+    assert!(ps1.contains("Add-UserPath $bindir"));
+}
+
 #[test]
 fn unix_installer_rejects_unsafe_source_update_cache_paths() {
     let install = include_str!("../install.sh");