Release v1.0.0 - Stable API with modular architecture

Major release with breaking changes:
- cfgmatic-core: Shared types (Config, AsyncConfig, Error, Change)
- cfgmatic-state: Terraform-like model (Plan, Apply)
- cfgmatic-loaders: Multiple loading strategies (Static, Watch, Multi, AsyncMulti)
- RuleBasedDiscovery: Advanced configuration file discovery
- Comprehensive documentation and test coverage