What

Replace all throw statements in public API headers with common::Result<T> return types to ensure consistent error handling before API freeze.

Part of #670

Why

• Public API currently mixes exception-based and Result-based error handling
• error_handling.h provides comprehensive make_error_result<T>() infrastructure but it's not consistently applied
• v1.0 API stability requires a single, predictable error handling pattern
• Throwing exceptions from a threading library can cause undefined behavior if not caught at thread boundaries

Where

Files Affected

Reference

• include/kcenon/thread/core/error_handling.h — error code enum and Result utilities

How

Technical Approach

1. For each throw site, identify the appropriate error_code from error_handling.h
2. Change return type from T to common::Result<T> (or common::VoidResult)
3. Replace throw with return make_error_result<T>(error_code::xxx)
4. Update call sites in tests to use .value() or error checking
5. Ensure backward compatibility through clear migration notes

Acceptance Criteria

• Zero throw statements remain in public API headers
• All modified functions return common::Result<T> or common::VoidResult
• Error codes are appropriate and documented in error_handling.h
• All existing tests updated and passing
• No new compiler warnings introduced