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>

CHANGELOG.md

@@ -28,6 +28,7 @@ Packaging, commands and lazy imports only; the formats stay at 0.1.0, and no com
 - The package version comes from git tags through setuptools-scm.
 - The base package has no dependencies and requires Python 3.14, the only version with the Unicode 16.0 character data the specification pins, and refuses to import under any other Unicode version; the extras `paperspec[schema]` (`jsonschema`, `referencing`), `paperspec[crypto]` (`cryptography`) and `paperspec[tools]` (both) cover `schema` and the HKDF and Ed25519 functions of `crypto`, which now import their packages on first use.
 - `paperspec.resources.path` finds the schemas, SQL and vectors in an installed wheel and in an editable install alike.
-- The programs in `tools/` become the package's commands in `src/paperspec/cli/`: `paperspec-check`, `paperspec-build`, `paperspec-validate`, `paperspec-load`, `paperspec-stress` and `paperspec-site`, which the pixi tasks and the workflow run.
+- The programs in `tools/` become subcommands of one command, `paperspec`, in `src/paperspec/cli/`: `paperspec validate`, `load`, `check`, `build`, `stress` and `site`, which the pixi tasks and the workflow run, and which `python -m paperspec` also runs.
+- `paperspec --help` says which commands work anywhere and which need a checkout, and each command's `--help` gives advice and examples; a misspelled command gets a suggestion.
 - `src/` is Apache-2.0, as `tools/` was.
 - The pixi workspace installs the package editable, and a `dev` environment builds the wheel with `pixi run -e dev wheel`.