refactor: reduce allocations in the sim task

[prestwich]

📝 Summary

Reduce cloning in the simulator task. This minimizes and defers new allocations. It is both code cleanup and perf. There are no logic changes.

✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[prestwich]

using the Cow allows us to defer the clone to here. This means the clone is eliminated in the (presumably common) case that simulation fails

[prestwich]

using the reference allows us to not clone the parents, saving a potentially very large allocation

[Copilot]

Pull request overview

This PR refactors the simulator task to reduce unnecessary cloning and defer allocations for better performance. The changes are primarily focused on passing references instead of owned values and using Cow<'_, Order> to defer cloning until the value needs to be owned.

Key changes:

• Function signatures for simulate_order and simulate_order_using_fork now accept &[Order] for parent orders and Cow<'_, Order> for the main order
• NonceKey struct changed from Clone to Copy since it only contains copy types
• Removed unnecessary .clone() calls throughout the codebase

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
simulation_job.rs	Updated comment about tokio::select fairness with documentation link
sim_worker.rs	Changed to pass references to parent orders and wrap owned order in Cow::Owned
sim.rs	Core refactoring: changed function signatures to accept references, added Cow for deferred cloning, removed unnecessary clones, added tracing instrumentation, and changed NonceKey to Copy

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