Update all references across docs, examples, and configuration files to
use the shorter `extras` feature name.

Add a `From<pack_Type> for crate::Entry` implementation in
`dispatcher!()` macro and a default `Entry` pack type in
`gen_program!()` for ergonomic program entry handling

Move pathf type resolution from `program_final_gen` to `gen_program` and
isolate generated code inside `__this_program_impl` to prevent namespace
pollution

program_final_gen

`just_fmt::pascal_case!`

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.

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.

Implement `RenderResult::append_other()` to merge buffered content and
immediate output behavior from one result into another. Add the
`r_append!` macro with both explicit buffer (`dst, src`) and implicit
buffer (`src`) usage forms, re-exported from `mingling::prelude`.