refactor: move the reference implementation to src/paperspec

The oracle had two import names, `tools.oracle` for the tools and
`paperspec` for other repositories, which could load the same code twice
in one process and needed a gate keyed on the module name. It now lives
in the standard src layout under its one name.

- git mv tools/oracle -> src/paperspec; the 22 rule modules are unchanged.
- The tools import `paperspec`. `tools/__init__.py` now holds the Python
  3.14 and cryptography/jsonschema check and puts src/ first on sys.path,
  so the tools run this checkout's code with or without pixi. The
  package's own __init__ checks nothing and exposes __version__.
- pyproject.toml: version from git tags via setuptools-scm
  (guess-next-dev, node-and-timestamp, src/paperspec/_version.py,
  gitignored). The packages stay listed rather than found because
  schemas/, sql/ and vectors/ stay at the root: the spec links to them and
  the site serves the schemas at their $id URLs.
- pixi.toml: a `dev` feature and environment with setuptools,
  setuptools-scm and python-build, and `pixi run -e dev wheel`.
- check.py scans src/paperspec for rule IDs; README, CONTRIBUTING, the
  FAQ, the overview, the vectors and tools READMEs and the changelog point
  at the new path. The FAQ no longer says the oracle is not a library.

pixi.lock is not re-solved in this commit: with a dynamic version, pixi
builds the package metadata to lock, which needs the conda environment
installed. Run `pixi lock` and commit the result before pushing; the site
workflow installs with --locked.

pixi run check passes; no computed value, vector, fixture, schema or SQL
file changed.

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

pixi.toml

@@ -20,11 +20,24 @@ markdownlint-cli2 = ">=0.23.3"
 nodejs = ">=22"
 cryptography = ">=46"
 
-# The tools import the oracle as `tools.oracle`; other programs install it as `paperspec`. The editable
-# self-install makes both resolve to the same code.
+# 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.
 [pypi-dependencies]
 paperspec = { path = ".", editable = true }
 
+# What building the package needs: pixi run -e dev wheel
+[feature.dev.dependencies]
+setuptools = ">=80"
+setuptools-scm = ">=8"
+python-build = ">=1.2"
+
+[feature.dev.tasks]
+wheel = { cmd = "python -m build --wheel --no-isolation --outdir dist", description = "Build the paperspec wheel into dist/" }
+
+[environments]
+default = { solve-group = "default" }
+dev = { features = ["dev"], solve-group = "default" }
+
 [tasks]
 check = { cmd = "python tools/check.py", depends-on = [
     "lint",