docs: add API reference with full type and method documentation

Phase 2 task: API documentation covering:
- All core types (User, Domain, Role, Object, ObjectData, Policy,
  UserRolePair, Directory, DirectoryRequest, DirectoryResponse)
- All four service interfaces (IBaseService, IFeatureService,
  IDirectoryService, ICurrentService) with parameters, return values,
  and usage notes for each method
- Constructor (New) and Options fields
- Utility functions (Check, Filter, ID, IDMap)
- Error reference table
- Update README docs table to link to api-reference.md

Author: AWaterColorPen

README.md

@@ -189,7 +189,7 @@ stage, err := playground.NewPlaygroundWithSqlitePath(t.TempDir())
 | [Getting Started](./docs/getting-started.md) | Step-by-step guide with complete runnable examples |
 | [Configuration](./docs/configuration.md) | All config options: DB, dictionary, watcher |
 | [Design](./docs/design.md) | Architecture and design decisions |
-| [API](./docs/api.md) | Full service method reference |
+| [API Reference](./docs/api-reference.md) | Full type and method reference with parameters, return values, and usage notes |
 
 ## Storage backends