fix(launcher): harden installer transactions

src/AcDream.Bake/BakeOutputTransaction.cs

@@ -9,6 +9,8 @@ namespace AcDream.Bake;
 /// </summary>
 public static class BakeOutputTransaction
 {
+    internal const string StagingMarker = ".acdream-bake.";
+
     public static TResult WriteValidateAndPublish<TResult>(
         string destinationPath,
         Func<string, TResult> writeTemporary,
@@ -25,9 +27,7 @@ public static class BakeOutputTransaction
             throw new InvalidOperationException("destination has no parent directory");
         Directory.CreateDirectory(directory);
 
-        string temporaryPath = Path.Combine(
-            directory,
-            $".{Path.GetFileName(fullDestination)}.{Guid.NewGuid():N}.tmp");
+        string temporaryPath = CreateStagingPath(fullDestination, Guid.NewGuid());
 
         try
         {
@@ -60,4 +60,21 @@ public static class BakeOutputTransaction
             }
         }
     }
+
+    /// <summary>
+    /// Exact adjacent staging-name contract shared, by documentation and
+    /// conformance tests, with Launcher.Core. Keeping this tiny contract in
+    /// each BCL-facing assembly avoids an otherwise inverted project edge.
+    /// </summary>
+    internal static string CreateStagingPath(string destinationPath, Guid transactionId)
+    {
+        string fullDestination = Path.GetFullPath(destinationPath);
+        string directory = Path.GetDirectoryName(fullDestination)
+            ?? throw new InvalidOperationException(
+                "destination has no parent directory");
+        return Path.Combine(
+            directory,
+            $".{Path.GetFileName(fullDestination)}{StagingMarker}"
+                + $"{transactionId:N}.tmp");
+    }
 }