Summary

This PR implements Phase 1 of the decentralized error category system to improve system isolation and reduce coupling between components.

Key Changes

• error_category: Abstract base class following std::error_category pattern for system-specific error categories
• common_error_category: Singleton implementation for common/shared error codes (success, invalid_argument, not_found, timeout, etc.)
• typed_error_code: Type-safe error code class that carries its category reference
• Helper functions: make_typed_error_code(), is_success(), is_error() for convenient error code creation and checking
• Result integration: error_info can now be constructed from typed_error_code for seamless integration

Design Decisions

• Named typed_error_code to avoid conflict with existing using error_code = error_info; alias
• Preserved backward compatibility with legacy error_code alias (marked deprecated)
• Categories use thread-safe singleton pattern via C++11 static local initialization
• Error codes carry both numeric value and category reference for clear origin identification

Architecture

error_category (abstract base)
    ├── common_error_category (singleton)
    │   └── codes: success, unknown_error, invalid_argument, not_found, ...
    └── [future] system-specific categories
        ├── network_error_category
        ├── database_error_category
        └── ...

typed_error_code
    ├── value(): int
    ├── category(): error_category&
    └── message(): string (from category)

Future Phases (not in this PR)

• Phase 2: Migrate common errors to common_error_category
• Phase 3: Create system-specific categories in dependent systems
• Phase 4: Deprecate centralized error_codes.h

Test Plan

• Unit tests for error_category base class functionality
• Unit tests for common_error_category implementation
• Unit tests for typed_error_code class with category support
• Integration tests with Result<T>
• Thread safety tests for singleton pattern
• All 113 existing tests pass
• Build succeeds on macOS with Clang

Closes #300