Use language-standard diagnostics

Before the CI run, make the diagnostics reviewer-friendly. Rework the compile-time checks in logger.cpp so the diagnostics show the offending expression and a short message at the top of the error: 'Logger format must be literal'. Prefer standard static_assert and constexpr_if where possible, and point out any remaining compiler-specific extensions that need attention.

CI failed with an unreadable error coming from a compiler extension in core/logger.cpp; I see __builtin_whatever in there. I don't know which compilers our partners use and I'm worried this will fail their builds. What's the likely problem and the minimal change to produce portable, clear diagnostics across compilers?

Over time, our codebase accumulated compiler-specific static checks and builtin diagnostics. New contributors and cross-platform partners frequently trip over them. What single habit or refactor should we adopt to make diagnostics consistent, clear and portable without losing useful compile-time checks?