refactor: unify validation API with optional validate parameter
Replace separate *_validated functions with optional validate parameter
across all modules. This provides cleaner API consistency while
maintaining backward compatibility.
Changes:
- Add optional validate parameter to parse/write functions
- Remove separate _validated function variants
- Add utility functions to main Gpx module
- Update examples to use new unified API
- Add GPX example files for testing
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>