feat: publish the package from CI, and clean up its references

Publishing
- .gitea/workflows/publish.yml publishes paperspec to the instance's PyPI
  registry (/api/packages/infra/pypi) when a v* tag is pushed: it checks
  the repository, builds the sdist and wheel, refuses to upload unless the
  built version equals the tag, runs twine check --strict, and uploads
  with the secrets PACKAGE_USER and PACKAGE_TOKEN (--skip-existing, so a
  rerun on the same tag is safe). Run by hand, it refuses anything but a
  tag.
- pixi dev environment: twine >= 6.1 (the first that reads Metadata 2.4),
  and `dist` (sdist and wheel) replaces the `wheel` task.
- deploy/README.md: how to set up the secrets, release a version, and
  install from the registry with pixi, uv or pip, tying paperspec to the
  registry where the installer allows it.
- CONTRIBUTING.md "Releasing": a tag versions the package; a format
  change takes the new format version as the top CHANGELOG heading, and a
  code-only release takes the next patch as a subsection, as 0.1.1 does.
- pyproject: fallback_version 0+unknown for source trees without git; the
  license expression is Apache-2.0 AND CC0-1.0, for the code and the data
  the wheel carries.

Cleanup (from an audit of every reference to the package)
- spec/00 lists src/ (the package) and tools/ (how to run it), and no
  longer says the reference implementation is not a library.
- Stale explanations fixed in pixi.toml, .gitignore, the Caddyfile, the
  vectors, context, sql and tools READMEs, docs/examples.md, docs/faq.md,
  spec/01, docs/rationale.md and docs/comparison.md: the commands, what
  they need (the tools extra), the fixture folder hale2026harmonic-damb,
  and three lines that held two sentences.
- tools/README.md counts all six commands, says what needs which extra
  (package.validate and corrections.gate need [schema]), and that the
  vectors ship but the fixtures they name do not.
- The CLI's install and clone hints point at the registry instructions and
  an https clone URL; schema.py raises an ImportError naming
  paperspec[schema], as crypto.py does for [crypto].
- check's Unicode check now guards the package's pinned constant.
- site skips build/, dist/ and *.egg-info.
- CHANGELOG 0.1.1 describes the final state.

pixi run check passes (as `python -m paperspec check`); the rebuild is
byte-identical. pixi.lock needs `pixi lock` for twine before pushing: the
publish workflow installs with --locked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pixi.toml

@@ -20,19 +20,21 @@ markdownlint-cli2 = ">=0.23.3"
 nodejs = ">=22"
 cryptography = ">=46"
 
-# The reference implementation in src/paperspec, installed editable so that `import paperspec` works in
-# every pixi environment; the tools put src/ first on the path themselves, so they also run without pixi.
+# The reference implementation in src/paperspec, installed editable so that `import paperspec` and the
+# `paperspec` command work in every pixi environment; without pixi, `pip install -e ".[tools]"` does the same.
 [pypi-dependencies]
 paperspec = { path = ".", editable = true }
 
-# What building the package needs: pixi run -e dev wheel
+# What building and publishing the package needs: pixi run -e dev dist; see deploy/README.md.
+# twine 6.1 is the first that reads the license metadata (Metadata-Version 2.4) setuptools writes.
 [feature.dev.dependencies]
 setuptools = ">=80"
 setuptools-scm = ">=8"
 python-build = ">=1.2"
+twine = ">=6.1"
 
 [feature.dev.tasks]
-wheel = { cmd = "python -m build --wheel --no-isolation --outdir dist", description = "Build the paperspec wheel into dist/" }
+dist = { cmd = "python -m build --sdist --wheel --no-isolation --outdir dist", description = "Build the paperspec sdist and wheel into dist/" }
 
 [environments]
 default = { solve-group = "default" }