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>

README.md

@@ -62,6 +62,7 @@ The address grammar is in [spec/06 §6.1](spec/06-anchors.md#61-addresses), and
 | [vectors/](vectors/README.md) | Test vectors: inputs and the exact outputs every implementation must compute. |
 | [fixtures/](fixtures/README.md) | Example packages and records, deliberately broken packages, and the test keys that signed them. |
 | [tools/](tools/README.md) | The programs that build and check everything above. |
+| [src/paperspec/](src/paperspec/) | The reference implementation of every rule, installable as the Python package `paperspec`. |
 | [docs/](docs/README.md) | Guides for people getting to know the project. |
 | [deploy/](deploy/README.md) | How the website at <https://paperspec.scient.ing> is built and served. |