Add clang-format configuration and formatting guidelines

[akrivx]

Description

Introduce a consistent code formatting setup using clang-format to improve readability and reduce formatting-only diffs.

Scope

• Add a .clang-format file at the repository root
• Choose a base style (e.g. LLVM) and customise as needed
• Document how to run clang-format locally (manual or via script)
• Optionally add a CI check to ensure formatting compliance

Motivation

• Keeps formatting consistent across contributors
• Reduces review noise
• Makes refactors and renames easier to review

Non-goals

• No enforcement via pre-commit hooks initially

Acceptance criteria

• clang-format can be run on the codebase using the provided config
• Formatting rules are documented briefly (README or CONTRIBUTING)