feat(hydro_lang): add sim_atomic_input API

[shadaj]

Summary

Adds Process::sim_atomic_input() which returns a SimAtomicSender<T> and an atomic stream. The sender's async send method resolves once the atomic tick has completed processing.

Implementation

Internally wraps values in Rc<RefCell<Option<T>>> to clone the stream without requiring T: Clone — one branch takes the value (returned to the user), the other maps to () and flows through end_atomic().sim_output() to signal completion.

Usage

let (sender, atomic_stream) = process.sim_atomic_input::<i32>();
// Use atomic_stream in your dataflow

// At runtime:
sender.send(42).await; // resolves after atomic tick completes

Closes #2772

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