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>

pixi.toml

@@ -39,13 +39,13 @@ default = { solve-group = "default" }
 dev = { features = ["dev"], solve-group = "default" }
 
 [tasks]
-check = { cmd = "paperspec-check", depends-on = [
+check = { cmd = "paperspec check", depends-on = [
     "lint",
 ], description = "Check schemas, records, packages, vectors, a byte-for-byte rebuild, the SQL round trip, prose and links" }
-quick = { cmd = "paperspec-check --quick", description = "Check everything except the byte-for-byte rebuild" }
-build = { cmd = "paperspec-build", description = "Rebuild fixtures/ and vectors/ from fixtures/src/ and the oracle" }
+quick = { cmd = "paperspec check --quick", description = "Check everything except the byte-for-byte rebuild" }
+build = { cmd = "paperspec build", description = "Rebuild fixtures/ and vectors/ from fixtures/src/ and the oracle" }
 lint = { cmd = "markdownlint-cli2", description = "Lint every Markdown file with markdownlint" }
-load = { cmd = "paperspec-load", description = "Load packages into SQLite: pixi run load --sqlite lab.db PACKAGE..." }
-validate = { cmd = "paperspec-validate", description = "Validate packages, or run proposed corrections through the gate" }
-stress = { cmd = "paperspec-stress", description = "Stress the oracle with random inputs: pixi run stress --seed N" }
-site = { cmd = "paperspec-site", description = "Build the website into _site/" }
+load = { cmd = "paperspec load", description = "Load packages into SQLite: pixi run load --sqlite lab.db PACKAGE..." }
+validate = { cmd = "paperspec validate", description = "Validate packages, or run proposed corrections through the gate" }
+stress = { cmd = "paperspec stress", description = "Stress the oracle with random inputs: pixi run stress --seed N" }
+site = { cmd = "paperspec site", description = "Build the website into _site/" }