feat(hydro_lang): add sim_atomic_input with scheduler support

[shadaj]

Summary

Adds Process::sim_atomic_input() which returns a SimAtomicSender<T, O, R> and a Stream<T, Atomic<Process>, ...>. The sender's send_atomic method is synchronous — values are immediately available in the next atomic slice without requiring a separate tick or ack pattern.

The sim scheduler now properly explores both orderings when an atomic input and a regular input are sent together.

Implementation

1. Source placement: The ExternalInput node is placed directly at the Atomic location (not at the top-level process with a batch into the tick).

2. AtomicSourceHook: A new sim hook type that monitors the external input channel. When items are pending, can_make_nontrivial_decision() returns true, which forces the scheduler to consider the tick as ready. On release, all pending items are emitted (no batching choice).

3. Scheduler integration: The hook is registered in create_external_source when the source location is Atomic. Items from the external channel are buffered at the process level and released into the tick graph via the hook.

Usage

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

// At runtime — synchronous, no await needed:
sender.send_atomic(42);
sender.send_many_atomic([1, 2, 3]);

Tests

• sim_atomic_input_api: verifies basic send + scheduler explores both orderings (2 instances)
• sim_atomic_input_same_tick: verifies the scheduler explores the same-tick scenario
• Ported 4 request_response tests in hydro_std to use the new API

Closes #2380

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Pull request overview

Adds Process::sim_atomic_input() for the Hydro simulator, returning a SimAtomicSender paired with a Stream whose source is placed directly inside an Atomic<Process> location. A new AtomicSourceHook lets the scheduler treat pending atomic-input items as a non-trivial reason to advance the tick, enabling exhaustive exploration of both same-tick and later-tick interleavings between the atomic input and other inputs. Existing request_response tests are ported off the ack pattern to the simpler synchronous API.

Changes:

• New SimAtomicSender wrapper with synchronous send_atomic / send_many_atomic[_unordered] methods.
• New AtomicSourceHook and dedicated create_external_source branch in the sim builder that buffers items at the root location and releases them into the tick graph via the hook.
• New Process::sim_atomic_input constructor and sim_atomic_input_same_tick regression test; hydro_std::request_response tests migrated to the new API.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
hydro_lang/src/sim/mod.rs	Declares the SimAtomicSender wrapper type.
hydro_lang/src/sim/compiled.rs	Implements synchronous send_atomic / send_many*_atomic on SimAtomicSender.
hydro_lang/src/sim/runtime.rs	Adds AtomicSourceHook that drains all pending items on each scheduler decision.
hydro_lang/src/sim/builder.rs	Special-cases LocationId::Atomic in create_external_source to install the hook and bridge buffer→tick.
hydro_lang/src/location/process.rs	Adds Process::sim_atomic_input constructing an ExternalInput directly at the Atomic location.
hydro_lang/src/location/tick.rs	Adds sim_atomic_input_same_tick regression test asserting both interleavings are explored.
hydro_std/src/request_response.rs	Ports four tests from the ack pattern to sim_atomic_input / send_atomic.

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