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.

Generate `render()` and `do_chain()` match dispatch directly in
`program_final_gen`, using a compile-time `ASYNC_ENABLED` constant to
select the correct sync/async signature. Removes the
`__dispatch_program_renderers!` and `__dispatch_program_chains!` macros
from `mingling_core`.

Reduces indirection and allows taking ownership of the program instance
for proper cleanup before `process::exit()`