feat(launcher): add verified first-run installer

src/AcDream.Launcher.Core/Integrity/FileIntegrity.cs

@@ -27,6 +27,11 @@ public static class FileIntegrity
         return Convert.ToHexStringLower(hash);
     }
 
+    /// <summary>
+    /// Asynchronous, cancellable counterpart used while verifying a multi-
+    /// gigabyte prepared package. The file stays streamed and no buffer is
+    /// retained after the hash completes.
+    /// </summary>
     public static async Task<string> ComputeSha256HexAsync(
         string filePath,
         CancellationToken cancellationToken = default)
@@ -38,8 +43,8 @@ public static class FileIntegrity
             FileMode.Open,
             FileAccess.Read,
             FileShare.Read,
-            bufferSize: 4096,
-            useAsync: true);
+            bufferSize: 1024 * 1024,
+            options: FileOptions.Asynchronous | FileOptions.SequentialScan);
         byte[] hash = await SHA256.HashDataAsync(stream, cancellationToken)
             .ConfigureAwait(false);
         return Convert.ToHexStringLower(hash);