Partially apply clang-tidy fixes we don't enforce yet

[abadams]

Many of these come from clang-tidy-19, which we don't yet use

• Put a bunch of stuff into anonymous namespaces
• Delete some redundant casts (e.g. casting an int to int)
• Add some const refs to avoid copies
• Remove meaningless inline qualifiers on in-class definitions and constexpr functions
• Remove return-with-value from functions returning void
• Delete a little dead code
• Use std::min/max where appropriate
• Don't use a variable after std::forwarding it. It may have been moved from.
• Use std::string::empty instead of comparing length to zero

[steven-johnson]

Ready to land?