refactor: remove AsyncDecorator as it's an unnecessary abstraction

- Deleted async_decorator.hpp file
- Removed all references to AsyncDecorator from codebase
- Removed with_async() methods from IntegratorBuilder
- Updated documentation to reflect the removal
- Follows the philosophy: the best framework is no framework
- Users should use std::async directly for async needs

Author: WenyinWei

ARCHITECTURE_TRANSFORMATION_SUMMARY.md

@@ -44,7 +44,7 @@ Each facility focuses on **exactly one concern**:
 // ✅ GOOD: Each decorator has single responsibility
 class TimeoutDecorator     { /* ONLY timeout protection */ };
 class ParallelDecorator    { /* ONLY parallel execution */ };
-class AsyncDecorator       { /* ONLY async capabilities */ };
+
 class OutputDecorator      { /* ONLY output handling */ };
 class SignalDecorator      { /* ONLY signal processing */ };
 ```
@@ -57,7 +57,6 @@ Facilities combine **without dependencies**:
 auto integrator = make_builder(base)
     .with_timeout()     // Independent
     .with_parallel()    // Independent  
-    .with_async()       // Independent
     .with_signals()     // Independent
     .with_output()      // Independent
     .build();
@@ -79,7 +78,6 @@ public:
 #### 2. Independent Facilities
 - `TimeoutDecorator`: Timeout protection only
 - `ParallelDecorator`: Batch processing and Monte Carlo only
-- `AsyncDecorator`: Async execution only
 - `OutputDecorator`: Online/offline/hybrid output only
 - `SignalDecorator`: Signal processing only
 
@@ -89,7 +87,6 @@ class IntegratorBuilder {
 public:
     IntegratorBuilder& with_timeout(TimeoutConfig = {});
     IntegratorBuilder& with_parallel(ParallelConfig = {});
-    IntegratorBuilder& with_async(AsyncConfig = {});
     IntegratorBuilder& with_output(OutputConfig = {});
     IntegratorBuilder& with_signals(SignalConfig = {});
     std::unique_ptr<AbstractIntegrator<S, T>> build();
@@ -107,19 +104,19 @@ public:
 ```cpp
 // Any combination works
 auto research = make_builder(base).with_timeout().with_parallel().build();
-auto realtime = make_builder(base).with_async().with_signals().build();
+auto realtime = make_builder(base).with_timeout().with_signals().build();
 auto server = make_builder(base).with_timeout().with_output().build();
 auto ultimate = make_builder(base).with_timeout().with_parallel()
-                                  .with_async().with_signals()
+                                  .with_signals()
                                   .with_output().build();
 ```
 
 ### 3. Order Independence
 ```cpp
 // These are identical:
-.with_timeout().with_async().with_output()
-.with_output().with_timeout().with_async()
-.with_async().with_output().with_timeout()
+.with_timeout().with_parallel().with_output()
+.with_output().with_timeout().with_parallel()
+.with_parallel().with_output().with_timeout()
 ```
 
 ### 4. Unlimited Extensibility
@@ -149,7 +146,6 @@ auto research_integrator = make_builder(base_integrator)
 ```cpp
 auto control_integrator = make_builder(base_integrator)
     .with_timeout(TimeoutConfig{.timeout_duration = std::chrono::milliseconds{10}})
-    .with_async()
     .with_signals()
     .build();
 ```
@@ -172,7 +168,7 @@ auto interactive_integrator = make_builder(base_integrator)
             return !user_cancelled();
         }
     })
-    .with_async().with_signals().with_output()
+    .with_signals().with_output()
     .build();
 ```
 
@@ -264,7 +260,6 @@ auto integrator = make_builder(base).with_timeout().build();
 auto integrator = make_builder(base)
     .with_timeout(TimeoutConfig{...})
     .with_parallel(ParallelConfig{...})
-    .with_async(AsyncConfig{...})
     .with_signals(SignalConfig{...})
     .with_output(OutputConfig{...}, custom_handler)
     .build();

CI_DEBUG_SUMMARY.md

@@ -16,17 +16,12 @@ The CI workflows are failing due to template parameter issues in the composable
 - **Fix**: Removed the `T` template parameter and updated all method signatures
 - **Status**: ✅ COMPLETED
 
-### 3. AsyncDecorator
-- **File**: `include/core/composable/async_decorator.hpp`
-- **Fix**: Removed the `T` template parameter and updated all method signatures
-- **Status**: ✅ COMPLETED
-
-### 4. OutputDecorator
+### 3. OutputDecorator
 - **File**: `include/core/composable/output_decorator.hpp`
 - **Fix**: Removed the `T` template parameter and updated all method signatures
 - **Status**: ✅ COMPLETED
 
-### 5. SignalDecorator
+### 4. SignalDecorator
 - **File**: `include/core/composable/signal_decorator.hpp`
 - **Fix**: Removed the `T` template parameter and updated all method signatures, including the SignalInfo struct
 - **Status**: ✅ COMPLETED

COMPOSABLE_ARCHITECTURE.md

@@ -45,7 +45,6 @@ Instead, we use independent decorators that can be composed:
 auto integrator = make_builder(base_integrator)
     .with_timeout()
     .with_parallel()
-    .with_async()
     .with_signals()
     .with_output()
     .build();
@@ -89,15 +88,6 @@ class ParallelDecorator : public IntegratorDecorator<S, T> {
 };
 ```
 
-#### Async Facility
-```cpp
-class AsyncDecorator : public IntegratorDecorator<S, T> {
-    // ONLY handles async execution
-    std::future<void> integrate_async(state& s, T dt, T end);
-    std::future<void> step_async(state& s, T dt);
-};
-```
-
 #### Output Facility
 ```cpp
 class OutputDecorator : public IntegratorDecorator<S, T> {
@@ -124,7 +114,6 @@ class IntegratorBuilder {
 public:
     IntegratorBuilder& with_timeout(TimeoutConfig config = {});
     IntegratorBuilder& with_parallel(ParallelConfig config = {});
-    IntegratorBuilder& with_async(AsyncConfig config = {});
     IntegratorBuilder& with_output(OutputConfig config = {});
     IntegratorBuilder& with_signals(SignalConfig config = {});
     
@@ -142,9 +131,9 @@ auto timeout_integrator = make_builder(base_integrator)
     .with_timeout(TimeoutConfig{.timeout_duration = std::chrono::seconds{30}})
     .build();
 
-// Async + signals
+// Timeout + signals
 auto realtime_integrator = make_builder(base_integrator)
-    .with_async()
+    .with_timeout()
     .with_signals()
     .build();
 ```
@@ -159,7 +148,6 @@ auto ultimate_integrator = make_builder(base_integrator)
         .enable_progress_callback = true
     })
     .with_parallel(ParallelConfig{.max_threads = 8})
-    .with_async(AsyncConfig{.thread_pool_size = 4})
     .with_signals(SignalConfig{.enable_real_time_processing = true})
     .with_output(OutputConfig{
         .mode = OutputMode::HYBRID,
@@ -177,13 +165,13 @@ The decorator pattern ensures composition order doesn't matter:
 ```cpp
 // These are equivalent:
 auto integrator1 = make_builder(base)
-    .with_timeout().with_async().with_output().build();
+    .with_timeout().with_parallel().with_output().build();
 
 auto integrator2 = make_builder(base)
-    .with_output().with_timeout().with_async().build();
+    .with_output().with_timeout().with_parallel().build();
 
 auto integrator3 = make_builder(base)
-    .with_async().with_output().with_timeout().build();
+    .with_parallel().with_output().with_timeout().build();
 ```
 
 ## Benefits
@@ -247,7 +235,6 @@ auto research_integrator = make_builder(base)
 ```cpp
 auto control_integrator = make_builder(base)
     .with_timeout(TimeoutConfig{.timeout_duration = std::chrono::milliseconds{10}})
-    .with_async()
     .with_signals()
     .build();
 ```
@@ -273,7 +260,6 @@ auto interactive_integrator = make_builder(base)
             return !user_cancelled();
         }
     })
-    .with_async()
     .with_signals()
     .with_output()
     .build();

IMPROVEMENTS_SUMMARY.md

@@ -26,7 +26,6 @@ class ParallelTimeoutAsyncSignalIntegrator { ... };
 auto integrator = make_builder(base_integrator)
     .with_timeout()     // Independent facility
     .with_parallel()    // Independent facility  
-    .with_async()       // Independent facility
     .with_signals()     // Independent facility
     .with_output()      // Independent facility
     .build();           // N classes for N facilities
@@ -43,7 +42,6 @@ include/core/composable/
 ├── integrator_decorator.hpp      # Base decorator pattern
 ├── timeout_decorator.hpp         # Timeout protection only
 ├── parallel_decorator.hpp        # Parallel execution only
-├── async_decorator.hpp          # Async execution only
 ├── output_decorator.hpp         # Output handling only
 ├── signal_decorator.hpp         # Signal processing only
 └── integrator_builder.hpp       # Composition builder
@@ -66,19 +64,13 @@ include/core/composable/
 - **Hardware Detection**: Automatic thread count detection
 - **Graceful Fallback**: Falls back to sequential on errors
 
-### 3. AsyncDecorator Capabilities
-- **Operation Tracking**: Active operation counting and monitoring
-- **Cancellation Support**: Cooperative cancellation mechanism
-- **Progress Monitoring**: Optional progress monitoring with callbacks
-- **Resource Management**: RAII scope guards for proper cleanup
-
-### 4. OutputDecorator Features
+### 3. OutputDecorator Features
 - **Multiple Modes**: Online, offline, and hybrid output
 - **File Output**: Optional file output with compression support
 - **Statistics**: Detailed performance monitoring and statistics
 - **Buffer Management**: Configurable buffering with overflow handling
 
-### 5. SignalDecorator Enhancements
+### 4. SignalDecorator Enhancements
 - **Multiple Processing Modes**: Synchronous, asynchronous, and batch
 - **Priority System**: Signal priority handling with queues
 - **Thread Safety**: Mutex protection for concurrent signal access
@@ -150,7 +142,6 @@ auto research_integrator = make_builder(base_integrator)
 ```cpp
 auto control_integrator = make_builder(base_integrator)
     .with_timeout(TimeoutConfig{.timeout_duration = std::chrono::milliseconds{10}})
-    .with_async()
     .with_signals()
     .build();
 ```
@@ -173,7 +164,7 @@ auto interactive_integrator = make_builder(base_integrator)
             return !user_cancelled();  // Allow cancellation
         }
     })
-    .with_async().with_signals().with_output()
+    .with_signals().with_output()
     .build();
 ```
 
@@ -225,7 +216,6 @@ auto ultimate_integrator = make_builder(base)
     .with_gpu()        // New facility
     .with_network()    // New facility  
     .with_compression() // New facility
-    .with_async()
     .with_signals()
     .with_output()
     .build();

docs/ASIO_INTEGRATION_GUIDE.md

@@ -12,7 +12,7 @@
 - **可视化**：生成图表或动画
 - **并行计算**：启动多个相关的 ODE 计算
 
-传统的做法是创建自定义的异步组件（如 `AsyncDecorator`），但这会导致：
+传统的做法是创建自定义的异步组件，但这会导致：
 
 1. **重复发明轮子**：重新实现队列、线程池、协程等基础设施
 2. **维护负担**：需要处理复杂的异步编程细节
@@ -219,17 +219,6 @@ asio::thread_pool pool(4);
 
 ## 与现有组件的对比
 
-### AsyncDecorator vs AsioIntegrationManager
-
-| 特性 | AsyncDecorator | AsioIntegrationManager |
-|------|----------------|------------------------|
-| 线程管理 | 自定义线程池 | 使用 asio 线程池 |
-| 任务调度 | 自定义队列 | asio 事件循环 |
-| 协程支持 | 无 | 完整支持 |
-| 错误处理 | 基本 | 完整 |
-| 资源管理 | 手动 | 自动 |
-| 学习成本 | 高 | 低（标准 asio） |
-
 ### 推荐使用场景
 
 **使用 AsioIntegrationManager 当：**
@@ -238,10 +227,10 @@ asio::thread_pool pool(4);
 - 需要与现有 asio 代码集成
 - 希望减少代码维护负担
 
-**使用 AsyncDecorator 当：**
-- 需要积分器内部的异步化
-- 对 asio 有特殊限制
-- 需要更细粒度的控制
+**直接使用标准库 std::async 当：**
+- 只需要简单的异步执行
+- 不需要复杂的任务编排
+- 希望最小化依赖
 
 ## 最佳实践