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/walkthrough.md

@@ -13,7 +13,7 @@ It has nothing to do with the repository's name.
 
 Open [fixtures/packages/hale2026harmonic-damb/](../fixtures/packages/hale2026harmonic-damb/) beside this page, because every ID below is in it.
 A package is a folder of files, mostly JSON Lines (`.jsonl`: one JSON record per line).
-The hand-written source of the first reading is [fixtures/src/hale2026harmonic.json](../fixtures/src/hale2026harmonic.json); no converter ran on it, and [`paperspec-build`](../src/paperspec/cli/build.py) expands it into the package.
+The hand-written source of the first reading is [fixtures/src/hale2026harmonic.json](../fixtures/src/hale2026harmonic.json); no converter ran on it, and [`paperspec build`](../src/paperspec/cli/build.py) expands it into the package.
 Each step says what a converter, a person or a checker does.
 
 ## 1. A PDF arrives
@@ -166,7 +166,7 @@ It is a [Frictionless Data Package](https://datapackage.org/standard/data-packag
 Its CSV keeps every digit as printed (`1.526` and `1.5260` stay different), and every value has an anchor to the cell that prints it, so a claim that rests on a value can show the cell.
 The descriptor types the fields as numbers; how a generic Data Package tool treats `1.5260` was not verified.
 All twelve value anchors resolve as `current`.
-The reference checker, [`paperspec-check`](../src/paperspec/cli/check.py), confirms each CSV value equals its cell's text.
+The reference checker, [`paperspec check`](../src/paperspec/cli/check.py), confirms each CSV value equals its cell's text.
 
 ## 8. Sharing without the text