[phase 3] Add .clang-format configuration to enforce consistent code style

[aregtech]

Important

Refactoring changes should be rebased and merged against this branch:
feature/655-api-modernization-and-refactoring

Summary

Add a .clang-format file to the repository root to enforce consistent formatting across all C++ source files. This should be done after the naming refactoring is complete, so the format reflects the final coding style.

Requirements

The .clang-format should enforce:

• C++17 standard
• Consistent brace style (Allman or K&R — to be decided based on current codebase majority)
• 4-space indentation (matching current code)
• Column limit (120 or 140, matching existing style)
• Pointer/reference alignment: const String & name style currently used
• Include sorting and grouping

Deliverables

• .clang-format file at repository root
• .clang-format-ignore if any files should be excluded (e.g., generated code, third-party)
• Verification: running clang-format on a sample of files produces reasonable output
• Optional: CI step to check formatting

Depends on

• All naming convention changes should be finalized first

Acceptance criteria

• .clang-format committed to repository root
• Format is consistent with the established code style
• No formatting conflicts with existing code patterns