Create testing helpers and documentation

[DrEverr]

Create a dedicated testing helpers module that provides:

1. Create TestJam class as easier discoverable API
2. New file: packages/jammin-sdk/testing-helpers.ts with:

• Re-export TestJam class with JSDoc examples
• Helper functions for common test scenarios:
  • expectAccumulationSuccess(result) - Custom assertions
  • expectStateChange(before, after, predicate) - State diff helpers

2. Documentation:

• Add comprehensive JSDoc to TestJam class methods
• Create testing guide in docs/testing.md
• Add code examples for common patterns

Acceptance Criteria:

• testing-helpers.ts created and exported from SDK
• All helpers have JSDoc with examples
• TestJam class with comprehensive documentation
• Testing guide covers: setup, writing tests, common patterns, troubleshooting

Files to Create/Modify:

• packages/jammin-sdk/testing-helpers.ts (new)
• packages/jammin-sdk/simulator.ts - enhance create TestJam with JSDoc
• packages/jammin-sdk/index.ts - export testing-helpers
• docs/testing.md (new)