Define and enforce v2 C++ coding style

[tkilias]

Goal

Define a public, usable C++ coding style for the v2 module and enforce its naming conventions automatically.

Scope

• Add public v2 coding-style guidance.
• Configure clang-tidy naming checks for classes, functions, variables, parameters, members, and namespaces.
• Migrate affected v2 identifiers to the documented convention.
• Validate the result with the full Bazel clang-tidy and clang-format checks.

Acceptance criteria

• The coding-style guide contains no internal-only guidance.
• The naming policy is enforced by clang-tidy.
• v2 sources comply with the policy.
• Formatting and static-analysis checks pass.