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/comparison.md

@@ -428,7 +428,7 @@ Keep using it as a reading and distribution format.
 *Status.*
 The specification is version 0.1.0.
 `registry/formats.json` lists 32 formats: 19 are `candidate` (complete, with vectors and fixtures, ready to implement) and 13 are `provisional` (complete; expected to change as implementations gain experience): project, bundle, dataset, digitization, view, snapshot, draft-ids, exercise, submission, grade, checklist, log-entry and checkpoint ([spec/00 §0.4](../spec/00-introduction.md#04-status)).
-The repository holds 24 chapters, 42 JSON Schemas, 25 vector files, five valid fixture packages, one of them also zipped, and 31 invalid ones, each naming the rule it breaks, and a reference checker, `paperspec-check`.
+The repository holds 24 chapters, 42 JSON Schemas, 25 vector files, five valid fixture packages, one of them also zipped, and 31 invalid ones, each naming the rule it breaks, and a reference checker, `paperspec check`.
 Licenses: prose CC BY 4.0; schemas, SQL, the JSON-LD context, registries, vectors and fixtures CC0 1.0; tools Apache-2.0 ([README.md](../README.md#licenses)).
 
 *No adoption claim.*