trait

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.

Add convenience presets (`mini`, `advanced`, `full`, `build_advanced`,
`build_full`) to `mingling/Cargo.toml` and document them in both
English and Chinese feature docs. Update getting-started guides to
reflect the new default `core`/`macros` features.

Add `Entry` struct with `pub(crate)` visibility for
`inner` fields, plus `new()` and `this()` constructors
on `ThisProgram` for docs.rs demonstration of the
`gen_program!` macro expansion.

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