[Testing] Add integration tests

[copyleftdev]

Task

Create comprehensive integration tests.

Test Cases

1. Parse all example Genesis files
2. Determinism: same seed = same output
3. Constraint enforcement: invalid states rejected
4. CLI command execution
5. Error message quality

Acceptance Criteria

• Tests in tests/ directory
• Use insta for snapshot testing
• All examples compile and generate
• CI-ready (no external dependencies)