Summary

• Add comprehensive docs/guides/ASYNC_OPERATIONS.md (~1000 lines) covering the async database operations module
• Update docs/API_REFERENCE.md with expanded Async Operations section and cross-references

What's Documented

Async Execution Model

• Architecture diagram showing execution layers
• thread_system integration with compile-time adapter pattern
• Performance comparison: thread_system (1.16M+ jobs/s) vs std::thread fallback (~50K jobs/s)
• Type aliases from thread_pool_adapter.h

async_executor

• Construction (thread count, context), task submission with SubmittableTask concept
• Lifecycle management: shutdown(), wait_for_completion(), pending_tasks()
• Performance characteristics table

async_result<T>

• Blocking operations: get(), get_for(timeout)
• Non-blocking operations: is_ready(), wait_for(timeout)
• Callback support: then() / on_error() with C++20 concept constraints
• Helper functions: make_ready_result(), make_error_result()

async_database

• Query operations: execute_async(), select_async()
• Batch operations: execute_batch_async(), select_batch_async()
• Async transaction operations: begin/commit/rollback
• Async connection management

C++20 Coroutine Support

• database_awaitable<T> with promise_type, await_ready/suspend/resume
• Coroutine methods: execute_coro(), select_coro()
• Helpers: when_all(), when_any()

Stream Processing

• stream_processor with 4 stream types (PostgreSQL NOTIFY, MongoDB Change Streams, Redis Pub/Sub, custom)
• stream_event structure
• Event handlers and filters with StreamEventHandler/StreamEventFilter concepts

Distributed Transactions

• transaction_coordinator with transaction state machine
• Two-Phase Commit protocol: prepare_phase(), commit_phase()
• Saga pattern: saga_builder with forward actions and compensations

Cross-Cutting Concerns

• Thread safety table for all classes
• Error handling patterns: future-based, callback-based, timeout
• Performance guide: async vs sync decision matrix, pool sizing, batching, backpressure

Examples

• Basic async queries
• Stream processing with filters
• Distributed transaction with Saga
• Coroutine-based workflow

Test Plan

• Markdown renders correctly (headings, tables, code blocks, diagrams)
• All cross-document links resolve (API_REFERENCE.md, ARCHITECTURE.md, UNIFIED_SYSTEM.md)
• Code examples match actual source API signatures
• Build succeeds with no regressions

Verification Results

Closes #356