Add CheckForUpdateAsync method for manual update checks and update docs

AlwaysUpToDate/DownloadManager.cs

@@ -138,7 +138,7 @@ namespace AlwaysUpToDate
                 updateTimer.Start();
             }
 
-            UpdateTimer_Elapsed(null, null);
+            _ = CheckForUpdateInternalAsync();
         }
 
         /// <summary>
@@ -166,7 +166,25 @@ namespace AlwaysUpToDate
             }
         }
 
+        /// <summary>
+        /// Manually performs a single update check against the remote manifest.
+        /// Raises <see cref="UpdateAvailable"/> or <see cref="NoUpdateAvailable"/> based on the result.
+        /// If an update is already in progress, the call is ignored.
+        /// </summary>
+        /// <returns>A task that represents the asynchronous check operation.</returns>
+        /// <exception cref="ObjectDisposedException">The updater has been disposed.</exception>
+        public async Task CheckForUpdateAsync()
+        {
+            ThrowIfDisposed();
+            await CheckForUpdateInternalAsync();
+        }
+
         private async void UpdateTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
+        {
+            await CheckForUpdateInternalAsync();
+        }
+
+        private async Task CheckForUpdateInternalAsync()
         {
             if (Interlocked.CompareExchange(ref updating, 0, 0) != 0)
             {

README.md

@@ -143,6 +143,13 @@ private static async void Updater_UpdateAvailable(string version, string changel
     }
 }
 ```
+### Manually checking for updates
+Use `CheckForUpdateAsync()` to trigger a single update check at any time, independently of the timer interval.
+`UpdateAvailable` or `NoUpdateAvailable` will be raised as usual when the check completes.
+```cs
+await updater.CheckForUpdateAsync();
+```
+This is also useful when the `Updater` is created without a timer interval or with `onlyUpdateOnce: true`, giving full control over when checks occur.
 ### Update process reporting
 ```cs
 private static void Updater_ProgressChanged(UpdateStep step, long itemsProcessed, long? totalItems, double? progressPercentage)