Add comprehensive validation for StaticConfig and DynamicConfig:
- ValidationError enum with thiserror for descriptive error messages
- validate() function that collects ALL errors (doesn't stop at first)
- All 18 validation rules from config.md implemented
- OR logic for allow_wildcard_bind (config OR CLI flag)
- Hostname normalization to lowercase during validation
- File existence check for manual mode cert_path and key_path
- Unit tests covering each validation rule with valid/invalid inputs
- Updated ConfigReloadHandle to use new validate() function
- Added PartialEq derives to config structs for diff_static_config
- Add [lib] target to enable integration test imports
- Add rcgen and reqwest dev-dependencies for TLS and HTTP test helpers
- Create src/config/test_fixtures.rs with test_static_config() and test_dynamic_config()
- Create tests/ with integration tests, HTTP test helper (TestUpstream), and TLS test helper (SelfSignedCert)
- Add Clone derives to StaticConfig and related structs for test fixture construction
- All existing tests continue to pass
