ci(docs): add contract event documentation drift validator (#714)

[ravendevhub]

Summary

Resolves #714 by introducing an automated documentation drift checker (scripts/validate-contract-events.py) and comprehensive event catalog (docs/CONTRACT_EVENTS.md) that parses Rust source files to ensure all 34 public Soroban contract events and their topics/data payloads remain synchronized with documentation.

Changes

1. Event Drift Validator (scripts/validate-contract-events.py):
   • Parses contract/contracts/hello-world/src/base/events.rs for #[contractevent] struct definitions.
   • Cross-references event names, topics, and payload fields against documentation.
   • Detects drift and exits with actionable warnings.
2. Contract Event Taxonomy (docs/CONTRACT_EVENTS.md):
   • Documents all 34 contract events including group lifecycle, financial, administrative, and notification events.
3. CI Integration (.github/workflows/validate-contract-events.yml):
   • Runs automatically on pull requests modifying contract events.

Acceptance Criteria

• Documented event names are validated.
• Required event fields are checked.
• Documentation drift can be detected during development or CI.