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>

fixtures/README.md

@@ -35,7 +35,7 @@ A conforming reader accepts each one and recomputes every hash it contains.
 ## Records
 
 `records/` holds a valid example of every other format that is a JSON file of its own.
-Three formats have their example elsewhere: the stage requirements (`scienting-stages/1`) and the stamp rules (`scienting-stamps/1`) are in [registry/](../registry/README.md), and a bundle (`scienting-bundle/1`) is a SQLite file that `paperspec-load --bundle` builds from the packages.
+Three formats have their example elsewhere: the stage requirements (`scienting-stages/1`) and the stamp rules (`scienting-stamps/1`) are in [registry/](../registry/README.md), and a bundle (`scienting-bundle/1`) is a SQLite file that `paperspec load --bundle` builds from the packages.
 Each record is a single file, or a folder where a format needs several files.
 
 | Files | What they are for |
@@ -109,4 +109,4 @@ Use them for nothing else.
 ## Sources
 
 `src/` holds the hand-written descriptions of the three main documents, as first read; see [src/README.md](src/README.md).
-Everything else here is generated by `paperspec-build`, so change the sources or the builder, never the generated files.
+Everything else here is generated by `paperspec build`, so change the sources or the builder, never the generated files.