Add comprehensive documentation for examples/folly

This commit adds a detailed README.md for the Folly integration example,
covering:

- Bidirectional async calls between Rust and C++
- Parallel computation with both coroutines and futures
- Exception handling across language boundaries
- Stream processing and complex communication patterns
- Complete build requirements and troubleshooting guide
- Comprehensive test coverage documentation

The documentation provides clear setup instructions, architecture
overview, and practical examples for developers wanting to understand
Folly-based async interoperability.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: NieR

examples/folly/README.md

@@ -0,0 +1,265 @@
+# Folly Integration Example
+
+This example demonstrates comprehensive async interoperability between Rust and C++ using the [Folly](https://github.com/facebook/folly) library. It showcases bidirectional async function calls, parallel computation, exception handling, stream processing, and complex communication patterns.
+
+## Overview
+
+The Folly example is a sophisticated demonstration of Rust-C++ async integration that covers:
+
+- **Bidirectional Async Calls**: Both Rust calling C++ async functions and C++ calling Rust async functions
+- **Parallel Computation**: Multithreaded dot product calculation using both Folly coroutines and futures
+- **Exception Handling**: Proper propagation of exceptions across language boundaries
+- **Stream Processing**: Async stream generators (FizzBuzz implementations)
+- **Complex Communication**: Ping-pong pattern with recursive async calls
+- **Resource Management**: Future dropping and coroutine lifecycle management
+
+## Prerequisites
+
+### System Requirements
+
+- **C++20 compiler** (GCC 10+, Clang 11+, or MSVC 2019+)
+- **Folly library** installed and available
+- **Rust 2018 edition** or later
+
+### Folly Installation
+
+The build script uses the `find-folly` crate to automatically locate your Folly installation. Ensure Folly is properly installed on your system:
+
+#### Ubuntu/Debian
+```bash
+sudo apt-get install libfolly-dev
+```
+
+#### macOS (with Homebrew)
+```bash
+brew install folly
+```
+
+#### Building from Source
+Follow the [official Folly installation guide](https://github.com/facebook/folly#build) for your platform.
+
+## Building and Running
+
+### Build the Example
+```bash
+cd examples/folly
+cargo build
+```
+
+### Run the Interactive Demo
+```bash
+cargo run
+```
+
+### Run Tests
+```bash
+cargo test
+```
+
+## Architecture
+
+### Key Components
+
+#### Rust Side (`src/main.rs`)
+- **Future Types**: `RustFutureVoid`, `RustFutureF64`, `RustFutureString`, `RustFutureStringNamespaced`
+- **Stream Types**: `RustStreamString`
+- **Parallel Computation**: Recursive async dot product using `async-recursion`
+- **Thread Pool**: Shared `ThreadPool` for concurrent execution
+
+#### C++ Side (`src/folly_example.cpp`)
+- **Folly Coroutines**: Using `folly::coro::Task` for async operations
+- **Folly Futures**: Traditional future combinators with `folly::Future`
+- **Thread Pool**: `folly::CPUThreadPoolExecutor` for parallel work
+- **Exception Handling**: Custom exception types with proper propagation
+
+#### Headers (`include/folly_example.h`)
+- **Future Definitions**: `CXXASYNC_DEFINE_FUTURE` macros for type mapping
+- **Stream Definitions**: `CXXASYNC_DEFINE_STREAM` macros
+- **Custom Exception Handling**: Specialized `TryCatch` implementation
+
+## Features Demonstrated
+
+### 1. Bidirectional Async Calls
+
+**Rust → C++**:
+```rust
+// Call C++ coroutine from Rust
+let result = ffi::folly_dot_product_coro().await.unwrap();
+
+// Call C++ future combinator from Rust  
+let result = ffi::folly_dot_product_futures().await.unwrap();
+```
+
+**C++ → Rust**:
+```cpp
+// Call Rust async function from C++
+auto future = rust_dot_product();
+double result = co_await std::move(future);
+```
+
+### 2. Parallel Computation
+
+Both languages implement the same parallel dot product algorithm:
+- **Vector Size**: 16,384 elements  
+- **Split Threshold**: 32 elements
+- **Thread Pool**: 8 threads (C++), futures thread pool (Rust)
+- **Algorithm**: Recursive divide-and-conquer with async/await
+
+### 3. Exception Handling
+
+**C++ Exceptions → Rust**:
+```cpp
+// C++ throws custom exception
+throw MyException("kaboom");
+```
+
+```rust
+// Rust catches as CxxAsyncException
+match result {
+    Err(err) => assert_eq!(err.what(), "kaboom"),
+    Ok(_) => panic!("should have failed"),
+}
+```
+
+**Rust Errors → C++**:
+```rust
+// Rust returns error
+Err(CxxAsyncException::new("kapow".into()))
+```
+
+```cpp
+// C++ catches exception
+try {
+    co_await rust_function();
+} catch (const rust::async::Error& e) {
+    std::cout << e.what() << std::endl; // "kapow"
+}
+```
+
+### 4. Stream Processing
+
+Async stream generators demonstrating:
+- **Basic Streams**: FizzBuzz sequence generation
+- **Nested Async**: Streams that internally await other futures
+- **Exception Propagation**: Streams that throw exceptions mid-stream
+- **Resource Cleanup**: Proper stream termination and cleanup
+
+### 5. Complex Communication Patterns
+
+**Ping-Pong Pattern**:
+```rust
+fn rust_folly_ping_pong(i: i32) -> RustFutureString {
+    RustFutureString::infallible(async move {
+        format!("{}ping ", 
+            if i < 4 {
+                ffi::folly_ping_pong(i + 1).await.unwrap()
+            } else {
+                "".to_owned()
+            }
+        )
+    })
+}
+```
+
+This creates a recursive call chain: Rust → C++ → Rust → C++ → ... until termination.
+
+## Test Coverage
+
+The example includes comprehensive tests covering:
+
+### Core Functionality Tests
+- `test_rust_calling_cpp_synchronously_coro()` - Rust → C++ coroutine calls
+- `test_rust_calling_cpp_synchronously_futures()` - Rust → C++ future calls  
+- `test_rust_calling_cpp_on_scheduler()` - Scheduled execution
+- `test_cpp_calling_void_rust_synchronously()` - C++ → Rust void calls
+- `test_cpp_calling_rust_synchronously()` - C++ → Rust value calls
+- `test_cpp_calling_rust_on_scheduler()` - Scheduled Rust execution
+
+### Error Handling Tests
+- `test_cpp_async_functions_throwing_exceptions()` - C++ exception propagation
+- `test_rust_async_functions_returning_errors()` - Rust error propagation
+
+### Advanced Pattern Tests  
+- `test_ping_pong()` - Recursive async communication
+- `test_complete()` - Void future completion
+- `test_dropping_futures()` - Resource cleanup
+- `test_fizzbuzz()` - Basic stream processing
+- `test_indirect_fizzbuzz()` - Nested async streams
+- `test_streams_throwing_exceptions()` - Stream error handling
+- `test_dropping_coroutines()` - Coroutine lifecycle management
+
+## Implementation Notes
+
+### Thread Safety
+- All async operations are thread-safe
+- Shared thread pools coordinate work between languages
+- Proper synchronization for cross-language communication
+
+### Memory Management
+- Automatic resource cleanup for futures and streams
+- RAII patterns ensure proper destruction
+- No memory leaks in cross-language async operations
+
+### Performance Considerations
+- Minimal overhead for language boundary crossings
+- Efficient parallel computation with proper work distribution
+- Optimized for both throughput and latency
+
+### Namespace Support
+The example demonstrates C++ namespace mapping:
+```cpp
+CXXASYNC_DEFINE_FUTURE(::rust::String, foo, rust, bar, RustFutureStringNamespaced);
+```
+
+```rust
+#[cxx_async::bridge(namespace = foo::rust::bar)]
+unsafe impl Future for RustFutureStringNamespaced {
+    type Output = StringNamespaced;
+}
+```
+
+## Troubleshooting
+
+### Common Build Issues
+
+**Folly Not Found**:
+```
+error: Couldn't find the Folly library!
+```
+Solution: Ensure Folly is installed and `PKG_CONFIG_PATH` includes Folly's `.pc` file.
+
+**C++20 Support**:
+```
+error: coroutines are a C++20 extension
+```
+Solution: Verify your compiler supports C++20 and the `--std=c++20` flag is properly set.
+
+**Linker Errors**:
+```
+undefined reference to folly symbols
+```
+Solution: Ensure all Folly dependencies are properly linked. Check `find-folly` output.
+
+### Runtime Issues
+
+**Thread Pool Errors**: Ensure sufficient system resources for the configured thread pools.
+
+**Segmentation Faults**: Usually indicate improper future/stream lifecycle management. Review the test patterns for proper usage.
+
+## Contributing
+
+When modifying this example:
+
+1. **Maintain Test Coverage**: All new features must include comprehensive tests
+2. **Document Changes**: Update this README for any new functionality
+3. **Follow Patterns**: Use existing error handling and async patterns
+4. **Verify Cross-Platform**: Test on multiple platforms if possible
+
+## Related Examples
+
+- [`examples/cppcoro`](../cppcoro/README.md) - Similar patterns using cppcoro instead of Folly
+- [`examples/common`](../common/README.md) - Shared utilities and patterns
+
+## License
+
+This example is licensed under both MIT and Apache 2.0 licenses, following the same licensing as the parent project.