Add async / concurrent execution mode for DAG flows

[dgenio]

Summary

The README roadmap (v0.4) mentions async execution as a planned capability, but no issue currently tracks this work. This issue covers adding an asynchronous execution mode to FlowExecutor that can run independent steps concurrently when the flow topology (DAG) allows it.

Motivation

Today FlowExecutor runs steps sequentially. Once DAG-based flows land (#8, #9), many steps in a flow will be independent of each other and could execute in parallel, significantly reducing end-to-end latency for I/O-bound tool chains.

Scope

• Add an AsyncFlowExecutor (or an async flag on the existing executor) that uses asyncio to run independent DAG branches concurrently.
• Ensure ExecutionResult / StepRecord correctly capture wall-clock vs. per-step timing.
• Provide a synchronous wrapper (run() → asyncio.run(...)) so the simple API remains unchanged.
• Update documentation and add at least one example demonstrating concurrent execution.

Acceptance Criteria

1. Independent steps in a DAG flow execute concurrently (verified by wall-clock time < sum of step times).
2. Dependent steps still respect topological ordering.
3. Error handling behaves consistently with sync mode (fail-fast or continue based on policy).
4. ExecutionResult includes per-step start/end timestamps.
5. Existing synchronous tests continue to pass.

References

• README roadmap v0.4: "Async execution"
• Related: Implement execution checkpoints and determinism-level metadata #8 (DAG topology), Add conditional branching support with safe predicate evaluation #9 (conditional branching), Implement persistent registry adapters (protocol + file-based store) #16 (parallel step execution)

[dgenio]

Closing as superseded by #80

Issue #80 ("Add async execution mode for sync and async tool chains") provides a comprehensive async story that fully encompasses this issue's scope:

• Tool supports both sync and async fn callables
• FlowExecutor.execute_flow_async() method
• Mixed sync/async tool chains
• Foundation for parallel DAG execution
• Backward-compatible sync API preserved

#80 is at the same milestone (v0.4.0) but with broader scope covering async tools, mixed chains, and MCP SDK compatibility — making #57 redundant.

Closing to eliminate duplication. See #80 for canonical tracking.

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Closed during backlog review.