Rename the internal fallback type and its associated `ProgramCollect`
plumbing from `ErrorDispatcherNotFound`/`build_dispatcher_not_found` to
`EntryFallback`/`build_entry_fallback`, along with the generated enum
variant and pack type. Update all documentation, examples, and tests
accordingly.

std::io::Write import

Replace r_println!/r_print! macro usage across all example renderers
with
explicit RenderResult construction using std::io::Write, enabling more
flexible output handling and reducing reliance on macro-side effects.