Add a reproducible NeMo Gym example for comparing routing strategies

Motivation

Switchyard’s existing benchmark example shows how to evaluate routing with Harbor and coding-agent benchmarks. Switchyard also integrates with NeMo Gym, which supports a broader collection of benchmarks and agent harnesses, but the Switchyard repository does not currently include a runnable example of that workflow.

Adding one would help users:

• Evaluate routing strategies on benchmarks beyond Harbor’s coding-agent workloads.
• Compare a routed strategy with a fixed-model baseline using the same tasks.
• Reproduce the workflow directly from the Switchyard repository.
• Discover and validate the NeMo Gym integration without assembling commands from external documentation.

Proposed solution

Add benchmark/nemo_gym/ with a small, executable, version-pinned example that:

• Defines a fixed-model baseline and one routed strategy.
• Runs both against the same benchmark and task subset.
• Writes each condition to a separate output directory.
• Pairs rollouts before comparing rewards.
• Compares model tokens, classifier tokens, latency, and routing statistics.
• Explains the generated provenance and statistics artifacts.
• Links to the full NeMo Gym integration documentation.

Start by comparing the existing benchmark example with the proposed benchmark/nemo_gym/ directory and review the linked NeMo Gym integration documentation. The example should run both a fixed-model baseline and a routed strategy on the same task subset, produce separate outputs, pair rollouts for reward comparison, and explain the provenance and statistics artifacts.