✨ feat(*): 添加扫码枪启动探活、全局退出助手及 README

- 添加扫码枪串口启动探活，检测端口占用并更新 UI 状态
- 新增 ShutdownHelper 安全停止 Host 扩展方法
- 新增 README.md 项目说明文档
- 更新 WorkflowHostedService 启动探活逻辑
- 补充 ShutdownHelper 与 WorkflowHostedService 单元测试
- 优化 DashboardPage 与 SystemSettingsPage 界面布局
- 调整 ModbusTcpPlcService 监控镜像读取逻辑

tests/AxiOmron.PcbCheck.Tests/ShutdownHelperTests.cs

@@ -0,0 +1,149 @@
+using System.Collections.Concurrent;
+using AxiOmron.PcbCheck.Utils;
+using Microsoft.Extensions.Hosting;
+using Xunit;
+
+namespace AxiOmron.PcbCheck.Tests;
+
+/// <summary>
+/// 验证应用退出阶段的异步清理流程不会丢失原始线程上下文。
+/// </summary>
+public sealed class ShutdownHelperTests
+{
+    /// <summary>
+    /// 停止 Host 后，后续收尾回调应继续在捕获到的同步上下文线程上执行。
+    /// </summary>
+    /// <returns>异步测试任务。</returns>
+    [Fact]
+    public async Task StopHostAsync_ShouldResumeOnCapturedSynchronizationContext()
+    {
+        await SingleThreadSynchronizationContext.RunAsync(async () =>
+        {
+            int expectedThreadId = Environment.CurrentManagedThreadId;
+            int callbackThreadId = -1;
+            var host = new FakeHost();
+
+            await ShutdownHelper.StopHostAsync(
+                host,
+                TimeSpan.FromMilliseconds(200),
+                () => callbackThreadId = Environment.CurrentManagedThreadId);
+
+            Assert.True(host.StopAsyncCalled);
+            Assert.Equal(expectedThreadId, callbackThreadId);
+        });
+    }
+
+    private sealed class FakeHost : IHost
+    {
+        public IServiceProvider Services => throw new NotSupportedException();
+
+        public bool StopAsyncCalled { get; private set; }
+
+        public void Dispose()
+        {
+        }
+
+        public Task StartAsync(CancellationToken cancellationToken = default)
+        {
+            return Task.CompletedTask;
+        }
+
+        public async Task StopAsync(CancellationToken cancellationToken = default)
+        {
+            StopAsyncCalled = true;
+            await Task.Yield();
+        }
+    }
+
+    /// <summary>
+    /// 提供仅在单线程内串行泵送回调的同步上下文，便于验证 await 后的线程恢复行为。
+    /// </summary>
+    private sealed class SingleThreadSynchronizationContext : SynchronizationContext
+    {
+        private readonly BlockingCollection<(SendOrPostCallback Callback, object? State)> _queue = new();
+
+        /// <summary>
+        /// 在专用线程同步上下文中执行异步委托。
+        /// </summary>
+        /// <param name="action">待执行的异步委托。</param>
+        /// <returns>表示执行完成的任务。</returns>
+        public static Task RunAsync(Func<Task> action)
+        {
+            ArgumentNullException.ThrowIfNull(action);
+
+            var completionSource = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+            var thread = new Thread(() =>
+            {
+                var context = new SingleThreadSynchronizationContext();
+                SynchronizationContext.SetSynchronizationContext(context);
+                Task task;
+
+                try
+                {
+                    task = action();
+                }
+                catch (Exception ex)
+                {
+                    completionSource.SetException(ex);
+                    return;
+                }
+
+                task.ContinueWith(
+                    completedTask =>
+                    {
+                        if (completedTask.IsFaulted)
+                        {
+                            completionSource.SetException(completedTask.Exception!.InnerExceptions);
+                        }
+                        else if (completedTask.IsCanceled)
+                        {
+                            completionSource.SetCanceled();
+                        }
+                        else
+                        {
+                            completionSource.SetResult();
+                        }
+
+                        context._queue.CompleteAdding();
+                    },
+                    CancellationToken.None,
+                    TaskContinuationOptions.None,
+                    TaskScheduler.Default);
+
+                foreach ((SendOrPostCallback callback, object? state) in context._queue.GetConsumingEnumerable())
+                {
+                    callback(state);
+                }
+            })
+            {
+                IsBackground = true
+            };
+
+            thread.SetApartmentState(ApartmentState.STA);
+            thread.Start();
+            return completionSource.Task;
+        }
+
+        /// <summary>
+        /// 将委托异步排入当前上下文队列。
+        /// </summary>
+        /// <param name="d">待执行回调。</param>
+        /// <param name="state">状态对象。</param>
+        public override void Post(SendOrPostCallback d, object? state)
+        {
+            ArgumentNullException.ThrowIfNull(d);
+            _queue.Add((d, state));
+        }
+
+        /// <summary>
+        /// 在当前线程直接执行同步回调。
+        /// </summary>
+        /// <param name="d">待执行回调。</param>
+        /// <param name="state">状态对象。</param>
+        public override void Send(SendOrPostCallback d, object? state)
+        {
+            ArgumentNullException.ThrowIfNull(d);
+            d(state);
+        }
+    }
+}