Add constexpr enum tag extractor utility

[neysofu]

• Summary
  • document the new utility and related project context in AGENTS.md and its linked reference files
  • extend comptime runtime, flat imports, IR, and stdlib sources to expose an enum tag extractor usable in std and user code
  • wire the new helper through std import agnostic entrypoints and comptime helpers so enum variants can yield their enum type without data
• Testing
  • Not run (not requested)