[feat/precompile-pytypes] feat: move detail/class.h definitions to class-inl.h (#6133)

* 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

* fix: suppress -Wredundant-decls for class.h declarations

Several functions are forward-declared in headers that cannot include
class.h; GCC's -Wredundant-decls (used in CI cxx_flags) flags the
second declaration.

Assisted-by: ClaudeCode:claude-fable-5