feat: move detail/class.h definitions to class-inl.h

All 32 functions in detail/class.h are non-template plumbing (type/slot
machinery); move them out of line for the precompiled mode. Forward
declarations of these functions in other headers lose their inline
keyword to stay ODR-consistent in both modes.

Assisted-by: ClaudeCode:claude-fable-5
10 files changed