feat!: one paperspec command with subcommands, help and examples

The six console scripts (paperspec-check, -build, -validate, -load,
-stress, -site) become subcommands of a single `paperspec` command:
paperspec validate, load, check, build, stress and site. `python -m
paperspec COMMAND` runs the same thing.

- paperspec/cli/main.py builds the whole parser, apart from the commands,
  so `paperspec --help` and `paperspec COMMAND --help` work without a
  checkout or the tools extra; a command's module is imported only when
  it runs.
- `paperspec` with no command prints the overview: what each command does,
  examples, and which work anywhere (validate, load) and which need a
  checkout (check, build, stress, site). Each command's help has a
  description with advice, and worked examples; a misspelled command gets
  a suggestion (argparse suggest_on_error); `--version` prints the version
  and the pinned Unicode version.
- Each command module exposes run(args) and returns the exit status.
  Arguments that parse but do not fit together raise UsageError, which
  prints the command's usage and exits 2: load now also refuses
  --built-from or --store without --bundle.
- The failure messages say what to do: install paperspec[tools], clone the
  repository, rerun stress with the printed seed, or run paperspec build
  when the rebuild check differs. check and stress say how many failures
  they did not list.
- check runs the rebuild as `python -m paperspec build`.
- pyproject installs one script, `paperspec`; the pixi tasks and the docs
  use the subcommands.

pixi run check passes (run here as `python -m paperspec check` with src/ on
the path); the rebuild is byte-identical. pixi.lock still needs
`pixi lock`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

schemas/README.md

@@ -64,7 +64,7 @@ The `Registry` maps that name to the file's contents, so nothing is fetched over
 Never load a schema alone.
 
 Any draft 2020-12 validator works the same way: give it every schema under its `$id`, then validate against the one you need.
-`paperspec-check` validates every fixture with `jsonschema` this way, and checks that every schema keeps to the keyword subset.
+`paperspec check` validates every fixture with `jsonschema` this way, and checks that every schema keeps to the keyword subset.
 
 ## What a schema does not check