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>

CHANGELOG.md

@@ -24,7 +24,8 @@ The first release.
 
 Packaging and lazy imports only; the formats stay at 0.1.0, and no computed value, vector, fixture, schema or SQL file changed.
 
-- `tools/oracle` installs as the Python package `paperspec` (`pyproject.toml`, setuptools), with `schemas/`, `sql/` and `vectors/` as the data subpackages `paperspec.schemas`, `paperspec.sql` and `paperspec.vectors`, so other repositories import the reference implementation instead of reimplementing it.
+- The oracle moves from `tools/oracle/` to `src/paperspec/` and installs as the Python package `paperspec` (`pyproject.toml`, setuptools), with `schemas/`, `sql/` and `vectors/` as the data subpackages `paperspec.schemas`, `paperspec.sql` and `paperspec.vectors`, so other repositories import the reference implementation instead of reimplementing it.
+- The package version comes from git tags through setuptools-scm.
 - The base package has no dependencies and runs on Python 3.12; the extras `paperspec[schema]` (`jsonschema`, `referencing`), `paperspec[crypto]` (`cryptography`) and `paperspec[tools]` (both) cover `schema` and the HKDF and Ed25519 functions of `crypto`, which now import their packages on first use.
-- The Python 3.14 and package gate in `tools/oracle/__init__.py` runs only when the repository's own tools import it as `tools.oracle`.
-- The pixi workspace installs the package editable, so the tools and the package resolve to the same code.
+- The tools import `paperspec` from `src/`, and their Python 3.14 and package check moves to `tools/__init__.py`, so the package itself checks nothing on import.
+- The pixi workspace installs the package editable, and a `dev` environment builds the wheel with `pixi run -e dev wheel`.