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>

vectors/README.md

@@ -85,7 +85,7 @@ python3 tools/check.py --quick
 ```
 
 It needs Python 3.14 and two packages, `jsonschema` and `cryptography`, and has no option to run one file.
-It tests the reference implementation in `tools/oracle/` against the vectors.
+It tests the reference implementation in `src/paperspec/` against the vectors.
 It does not test yours.
 
 ## What each file tests
@@ -136,7 +136,7 @@ The specification, and the vectors with it, are at version 0.1.0 ([0.3](../spec/
 The specification has 32 formats: 19 are `candidate` and 13 are `provisional` in [registry/formats.json](../registry/formats.json).
 A candidate format changes only with a recorded reason and a new or changed vector ([22.5](../spec/22-conformance.md#225-stability)).
 
-A passing run shows one thing: for these inputs, your implementation computes the same values as the reference implementation in `tools/oracle/`.
+A passing run shows one thing: for these inputs, your implementation computes the same values as the reference implementation in `src/paperspec/`.
 That code computed the expected outputs, and `pixi run build` regenerates them ([tools/](../tools/README.md)).
 The vectors hold 311 cases in all, some of them small: `hashes.jsonl` has one document, `alignment.jsonl` one pair of readings, `offsets.jsonl` two cases.