Add struct unmarshaling support via JSON round-trip:
- Unmarshal() unmarshals full combined config into a struct
- UnmarshalKey() unmarshals a specific key (supports dot notation)
- Package-level convenience functions for default manager
- Comprehensive tests covering JSON, TOML, YAML, nested keys, scalars
- Bump Go to 1.26.2
- Fix stale Delete reference in README API table
- Add .gitignore for cover.out
- Add GetFloat64 and GetInt64 methods to ConfigManager and package-level API
- Fix README precedence: actual order is Set > env > file > default
(was incorrectly documented as file > env > default)
- Fix GetStringMap return type in API table: map[string]any, not map[string]string
- Bump Go to 1.26.1
- Add tests for all new getters (coverage 94.7% → 95.2%)
