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>

CONTRIBUTING.md

@@ -17,7 +17,7 @@ The schemas, SQL, registries, vectors and fixtures then follow the chapter ([spe
    Then run `pixi run build`, which rewrites `fixtures/` and `vectors/`.
 4. **Run the checks.**
    `pixi run check` must end with `All checks pass.`
-   Without pixi, install the checkout with `pip install -e ".[tools]"` on Python 3.14, then run `paperspec-check`.
+   Without pixi, install the checkout with `pip install -e ".[tools]"` on Python 3.14, then run `paperspec check`.
 5. **Record the change** in [CHANGELOG.md](CHANGELOG.md).
 
 ## Versions
@@ -39,7 +39,7 @@ Before tagging:
 
 1. `pixi run check` passes on Linux and macOS, which the workflow in `.gitea/workflows/` runs on every push.
    The check also confirms that every count the prose states (formats, schemas, vectors, broken packages) matches the repository, and that `pixi.toml`, the registries and `src/paperspec/cli/build.py` state the same version.
-2. `paperspec-stress` passes with its default seed, and with one or two others.
+2. `paperspec stress` passes with its default seed, and with one or two others.
 3. The website is published from the release ([deploy/](deploy/README.md)), and the JSON-LD context at `https://paperspec.scient.ing/context/scienting-1.jsonld` and a schema at its `$id` both answer with their media types.
    Layers claim to be W3C Web Annotations, and a JSON-LD processor that cannot fetch the context cannot read them.
 4. The decisions that freeze on release are made: enumeration values and format identifiers never change meaning afterwards ([spec/22 §22.5](spec/22-conformance.md#225-stability)), so a name worth changing is changed now.