feat: add executor pool support

[HuaHuaY]

No description provided.

[wgtmac]

Nice work. The abstraction is clean and the test coverage is solid. Since the default executor is nullopt, existing paths stay single-threaded, so the risk is well contained.

A couple of things before merge:

• The PR has no description. Worth adding the motivation and a short design note, ideally with the parallel/serial numbers that justify it.
• The RetryRunner change from a runtime fluent API (OnlyRetryOn/StopRetryOn) to the compile-time RetryRunner<Policy> is a breaking API change that's independent of executor support. Consider splitting it into its own PR so it gets reviewed on its own merits.

Left a few inline notes.

[wgtmac]

A few more minor notes, non-blocking.

[wgtmac]

Another pass, this time on the interface design and extensibility.

The overall shape is good: one virtual Executor that engines implement, threaded through the builders via PlanWith. The Arrow adapter test is a nice proof that an external pool drops in with basically one line, so "bring your own threadpool" is well covered.

On future async directions (C++23 coroutines, P2300 std::execution): the model here is synchronous and blocking, TaskGroup::Run() fans out and blocks on std::future::get(), and the planning APIs return Result<...> directly. So this is a parallel-for primitive, not a step toward a sender/receiver pipeline. That's a reasonable scope for now, I'd just flag it explicitly so nobody expects this interface to extend into async later, it'll be a separate one. Details inline.

[wgtmac]

One more pass. The main thing I found is a documentation gap around a new concurrency contract that now leaks onto user-supplied callbacks.

Once an executor is set, the user's ManifestWriterFactory and the shared FileIO get called from multiple worker threads. The tests already account for this (the factories use an atomic counter plus a barrier), so the requirement is understood, it's just not written down anywhere a downstream engine would see it. Worth documenting on the public surface.

[wgtmac]

We need to update LICENSE file