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`

Use the span of the `?` token when generating the macro invocation in
the `#[routeify]` transformation, so that rust-analyzer resolves hover
and go-to-definition on `?` to the `route!` macro's documentation

Restructure flat module layout into logical directories (attr/, derive/,
func/, systems/, extensions/, utils.rs). Tighten internal function
visibility to pub(crate). All public API signatures remain unchanged.

Implement an `extensions` module in `mingling_macros` that allows
attribute macros to accept extension identifiers (e.g., `routeify`)
which are re-dispatched as outer attributes before the core macro
logic runs, enabling extensibility without modifying existing macros

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