Add `Metadata<B>` trait and `#[metadata(Entry)]` attribute macro
to attach arbitrary, compile-time-typed metadata to entries.
Implement `ProgramCollect::get_metadata<T>()` for runtime retrieval,
backed by a global registry populated by `register_metadata!`.

Extend `pathf` with `MetadataPattern` to resolve metadata types
across modules at build time. Add two examples demonstrating
metadata usage with and without pathf integration.

Add module-level docs for the build module via include_str and
restrict public re-exports to explicit items instead of glob imports.

Replace the `builds` feature with `build` across the crate, keeping
a backward-compatible alias for the old name.  Update all internal
visibility from `#[doc(hidden)] pub mod` to `pub(crate) mod` and
reorganize the `__private` module into a dedicated `private.rs`.

Make `StructuralData` and `StructuralDataSealed` generic over a program
collector type to bypass the orphan rule for `group_structural!`. This
enables external types to implement `StructuralData<crate::ThisProgram>`
without violating coherence.

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.

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