clang-tidy cleanup part 1

[AJenbo]

Scope:

• Fix clang-tidy warnings
• Focus on one PR at a time with ~500 lines of diff to keep reviews manageable.

Instructions:

1. Pick a set of files/warnings that you can clean up within ~500 lines of diff.
2. Make a branch and fix these warnings.
3. Open a PR and reference this issue.
4. Repeat until all warnings in this scope are addressed.

Notes:

• Only fix warnings; do not introduce new ones.
• If a warning cannot be fixed safely, leave a comment and mark it with NOLINT.