[4/N] feat(sqsim): add scenario DSL and registry

[albertywu]

Summary

feat(sqsim): add scenario DSL and registry

Test Plan

Issues

Stack

1. [1/N] docs(sqsim): propose local scenario simulator #383
2. [2/N] docs(sqsim): define scenario and behavior model #384
3. [3/N] feat(orchestrator): publish request stage progress #385
4. @ [4/N] feat(sqsim): add scenario DSL and registry #386
5. [5/N] feat(sqsim): add modeled runtime primitives #387
6. [6/N] feat(sqsim): implement external system adapters #388
7. [7/N] feat(sqsim): wire modeled adapters into local stack #389
8. [8/N] feat(sqsim): run scenarios against local stack #390
9. [9/N] feat(sqsim): add interactive scenario view #391
10. [10/N] feat(sqsim): add functional scenario suite #392
11. [11/N] docs(sqsim): add user guide #393