Remove step pairing constraint and add parent_step_id for nested steps

[Zane-XY]

Problem: The current protocol constraint requiring StepStartedEvent and StepFinishedEvent to appear in pairs prevents rendering of nested steps—a common pattern where parent steps are divided into multiple sub-steps.

Example: Nested Steps (3 levels)

• Deploy Application (Level 1 - Root)
  • Build Phase (Level 2)
    • Compile source code (Level 3)
    • Run unit tests (Level 3)
    • Generate artifacts (Level 3)
  • Deploy Phase (Level 2)
    • Provision infrastructure (Level 3)
    • Deploy to staging (Level 3)
    • Run integration tests (Level 3)
    • Deploy to production (Level 3)
  • Verify Phase (Level 2)
    • Health check (Level 3)
    • Smoke tests (Level 3)
    • Monitor metrics (Level 3)

Solution:

• Remove the paired event verification constraint
• Add parent_step_id field to step events to establish parent-child relationships

This change will enable hierarchical step modeling while maintaining event traceability.

[NiuBlibing]

Good idea.