clap derive types replace the hand-written parser: typed values, usage
errors (exit 2) before any work, help from the same definitions.
Syntax changes: a repeated option is a usage error (was: last wins; the
automation roots still repeat); a value starting with `-` needs `=`; a
positional starting with `-` needs `--`; `--o` and the `version`
subcommand are gone (`--version` stays); `<sub> --help` prints that
subcommand's help; usage errors are clap's text; `--format` is checked
before the file is opened.
* Dev profile: line tables for workspace crates, no debug info for dependencies
* Tests: one integration-test binary per crate with several test files
* xtask ci: nextest, doctests, tests without examples, --keep-going; add xtask test
BFlat