fix: implement nested_steps/1 for Step.Switch to fix dependency resolution

[jimsynz]

Summary

• Implements nested_steps/1 callback for Reactor.Step.Switch
• Adds @impl true annotation to existing run/3 function (was missing)
• Adds integration tests that reproduce the issue with async execution
• Adds unit tests for the new nested_steps/1 callback

Root Cause

Step.Switch didn't implement nested_steps/1, so the planner couldn't see nested steps' dependencies. Without proper dependency edges:

1. The Switch step could run before its nested steps' dependencies completed
2. When nested steps were emitted at runtime, the planner couldn't find the dependency step
3. This caused the "step cannot be found" error

The existing test used async?: false which masked the race condition.

Test plan

• Added integration tests reproducing issue Step referenced from within a switch could not be found #273 with async?: true
• Added unit tests for nested_steps/1 callback
• All 331 tests pass
• All code quality checks pass (formatter, credo, dialyzer, etc.)

Closes #273