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>

docs/faq.md

@@ -107,7 +107,7 @@ A `candidate` format is complete, with vectors and fixtures, and ready to implem
 A `provisional` one is complete and expected to change as implementations gain experience.
 The provisional formats are dataset, digitization, view, project, bundle, snapshot, draft-ids, exercise, submission, grade, checklist, log-entry and checkpoint.
 
-The repository holds a reference checker (`paperspec-check`), 25 vector files, five valid packages, one of them also as a zip, and 31 deliberately broken packages ([spec/00 §0.4](../spec/00-introduction.md#04-status), [spec/22](../spec/22-conformance.md)).
+The repository holds a reference checker (`paperspec check`), 25 vector files, five valid packages, one of them also as a zip, and 31 deliberately broken packages ([spec/00 §0.4](../spec/00-introduction.md#04-status), [spec/22](../spec/22-conformance.md)).
 
 ## What does it not do?
 
@@ -126,6 +126,6 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md).
 
 ## What are the files in `tools/` for?
 
-`paperspec-build` builds the fixtures and vectors. `paperspec-check` recomputes and checks every one, and `paperspec-load` loads packages into SQLite.
+`paperspec build` builds the fixtures and vectors. `paperspec check` recomputes and checks every one, and `paperspec load` loads packages into SQLite.
 The oracle's modules in `src/paperspec/` compute every expected value, so that none is typed by hand.
 The other repositories of the project install them as the Python package `paperspec`; independent implementations follow the specification ([tools/README.md](../tools/README.md)).