Fix Phase 2: add [START]/[STEP]/[END] structured output + simulation fixes

Fix /reset to accept empty/null body for hackathon validator

Fix propagation recovery and config key visibility

Fix HTTP endpoint statefulness and response format

Add tests and fix premature termination bug

Add core simulation engine, environment, grader, and app wiring

Add propagation engine, log templates, and updated models

Checkpoint: existing implementation (pre-cleanup)