Add `#![deny(missing_docs)]` across multiple crates and fill in
documentation for all public items, including struct fields, enum
variants, trait methods, and proc macros.  Also mark two shell scripts
as executable.

macros

Reintroduce `r_print!` and `r_println!` macros as public exports, now
supporting both explicit buffer argument and implicit `#[buffer]` attr.
Add `#[buffer]` attribute macro that wraps unit-returning functions to
produce a `RenderResult` with an automatically injected buffer variable.
Relax `RenderResult::print()` and `println()` to accept `impl
AsRef<str>`.

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

methods

Deprecate `ShellContext` methods `filling_argument_first`,
`filling_argument`, `typing_argument`, `strip_typed_argument`, and
`get_typed_arguments` when the `picker` feature is active, as they
do not work under all `ParserStyle` settings.

Add `#![allow(deprecated)]` to the affected modules to suppress
warnings from internal usage.

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