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/overview.md

@@ -54,7 +54,7 @@ It appears inside hashed data and test vectors, and it is separate from the repo
 | [spec/](../spec/README.md) | The specification, in 24 chapters (00 to 23). Normative. |
 | [schemas/](../schemas/README.md) | 42 JSON (plain-text data format) Schemas for the files and records. |
 | [registry/](../registry/README.md) | The list of formats, the rule IDs a checker reports, download-stamp rules, and default rights requirements. |
-| [tools/](../tools/README.md) | How to run the reference commands: `paperspec-check` checks everything, `paperspec-build` rebuilds the fixtures and vectors, and `paperspec-load` loads packages into SQLite. |
+| [tools/](../tools/README.md) | How to run the reference commands: `paperspec check` checks everything, `paperspec build` rebuilds the fixtures and vectors, and `paperspec load` loads packages into SQLite. |
 | [src/paperspec/](../src/paperspec/) | The oracle, a compact implementation of every rule, which the other repositories of the project install as the Python package `paperspec`. |
 | [vectors/](../vectors/README.md) | 25 files of test vectors: inputs and the exact outputs every implementation must compute. |
 | [fixtures/](../fixtures/README.md) | Five valid example packages, one of them also as a zip, 31 deliberately broken ones that each break one named rule, example records, and the test keys that signed them. All text is synthetic. |
@@ -73,8 +73,8 @@ The repository ships reference tools only for the checker and store roles.
 | Reader | Opens a package, resolves addresses and anchors, and shows text, annotations and each passage's fidelity level. |
 | Writer | Produces files in these formats: nodes, corrections, receipts, annotations, datasets. |
 | Converter | Turns a witness (a PDF, JATS XML, the article format publishers use, a LaTeX source archive, an HTML page, an EPUB or a Typst project) into a package and raises doubtful readings as suspects (flagged disagreements awaiting review). |
-| Checker | Recomputes hashes, validates files against their schemas and reports broken rules by rule ID. `paperspec-check` does this for the repository. |
-| Store | Keeps packages and records in a database a team can clone. `paperspec-load` loads packages into SQLite. |
+| Checker | Recomputes hashes, validates files against their schemas and reports broken rules by rule ID. `paperspec check` does this for the repository. |
+| Store | Keeps packages and records in a database a team can clone. `paperspec load` loads packages into SQLite. |
 
 The [specification's reading paths](../spec/README.md) say which chapters each role needs.