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>

docs/overview.md

@@ -54,7 +54,8 @@ It appears inside hashed data and test vectors, and it is separate from the repo
 | [spec/](../spec/README.md) | The specification, in 24 chapters (00 to 23). Normative. |
 | [schemas/](../schemas/README.md) | 42 JSON (plain-text data format) Schemas for the files and records. |
 | [registry/](../registry/README.md) | The list of formats, the rule IDs a checker reports, download-stamp rules, and default rights requirements. |
-| [tools/](../tools/README.md) | Reference programs: `check.py` checks everything, `build.py` rebuilds the fixtures and vectors, `load.py` loads packages into SQLite, and `oracle/` is a compact implementation of every rule. |
+| [tools/](../tools/README.md) | Reference programs: `check.py` checks everything, `build.py` rebuilds the fixtures and vectors, and `load.py` loads packages into SQLite. |
+| [src/paperspec/](../src/paperspec/) | The oracle, a compact implementation of every rule, which the other repositories of the project install as the Python package `paperspec`. |
 | [vectors/](../vectors/README.md) | 25 files of test vectors: inputs and the exact outputs every implementation must compute. |
 | [fixtures/](../fixtures/README.md) | Five valid example packages, one of them also as a zip, 31 deliberately broken ones that each break one named rule, example records, and the test keys that signed them. All text is synthetic. |
 | [sql/](../sql/README.md) | Tables for SQLite, Dolt and PostgreSQL, and a SQLite bundle with full-text search. |