refactor(core) - Switch project to snake_case and switch project to std::error_code.

[raygoe]

Description

This PR refactors the entire project so that it uses snake_case and switches all the error objects for std::unexpected<T, E> to std::error_code which is the standard way of handling it.

Type of Change

• feat - New feature
• fix - Bug fix
• docs - Documentation update
• style - Code style changes (formatting)
• refactor - Code refactoring
• perf - Performance improvement
• test - Test additions/fixes
• build - Build system changes
• ci - CI configuration changes

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All tests pass locally

Code Quality

• Code follows the C++ Manual
• No compiler warnings
• Breaking changes documented (if any)
• Documentation updated

AI Policy Compliance

• No AI-generated content
• Contains AI-generated code (disclosed and reviewed)

Checklist

• Branch is up-to-date with main
• Commit messages follow conventional format
• CI checks pass
• Self-review completed