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.

Move routing methods exclusively to the `Routable` trait and update
all macro-generated code to reference `Routable` instead of `Grouped`

Introduce a `Routable` trait with `to_chain` and `to_render`
methods, replacing direct `Grouped` usage in the `route!` macro
so error branches correctly route through the rendering pipeline

Add module-level docs for the hook system, enhance doc examples for
macros
with usage syntax, fix typos in cross-references, and hide internal
debugging
macros and mock types from public documentation

Introduce `StructuralData` sealed trait and `pack_structural!` /
`group_structural!` / `derive(StructuralData)` macros to control
structured rendering separately from grouping. `Groupped` no longer
requires `Serialize`.

Extract duplicate `MockCollect` implementations into a reusable
`MockProgramCollect` type. Conditionally require `Serialize` on the
`Groupped` trait when the `general_renderer` feature is enabled.