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>

context/README.md

@@ -96,7 +96,7 @@ Three of the nine fixture annotations have only an `AnchorSelector` (the section
 - Reader: Follow [chapter 6](../spec/06-anchors.md) to resolve an anchor.
   The file `vectors/resolve.jsonl` resolves seventeen anchors against `fixtures/packages/hale2026harmonic-damb` and gives every outcome.
 - JSON-LD processor: Use a JSON-LD 1.1 processor; the context relies on `@json` and `@version` 1.1.
-- Checker: `schemas/annotation.schema.json` and `schemas/layer.schema.json` validate the files. `paperspec-check` also checks that each annotation's ID derives from its content and that the receipt at `fixtures/records/layer/receipt.json` covers each `confirmed` annotation.
+- Checker: `schemas/annotation.schema.json` and `schemas/layer.schema.json` validate the files. `paperspec check` also checks that each annotation's ID derives from its content and that the receipt at `fixtures/records/layer/receipt.json` covers each `confirmed` annotation.
 
 The traps: