fields

Declare `Grouped` as `unsafe trait` to make its soundness invariant
visible.
Make `AnyOutput.type_id` and `member_id` private; add public accessors
and
the `unsafe fn new_bare` constructor.

Update all documentation and code examples to use the new
`#[renderer(buffer)]`
pattern with `r_println!` instead of manually constructing
`RenderResult`
with `writeln!`.

Mark all tasks as complete and finalize the checklist