Simplify native aggregate initialization and lookups

[mentaljam]

Summary

• replace the C/C++ preprocessor branch used for generated struct returns with a shared positional aggregate initializer
• canonicalize semantic lookup rows to enum declaration order
• generate native lookup helpers as dense aggregate tables indexed by enum ordinal instead of switch statements
• keep a bounds check with an all-NAN fallback in C headers while C++ modules use direct enum-to-index lookup
• remove generated-code dependence on PTFKIT_RECORD_LITERAL, detail/record.h, and C++ std::unreachable() for lookups