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>

CONTRIBUTING.md

@@ -13,7 +13,7 @@ The schemas, SQL, registries, vectors and fixtures then follow the chapter ([spe
 2. **Change the schemas, SQL and registries** that the rule shapes.
    A new refusal gets a rule ID in [registry/rules.json](registry/rules.json).
 3. **Compute, never type, the expected values.**
-   Change the oracle in [tools/oracle/](tools/oracle/) to apply the rule, and add the case that shows it to [tools/build.py](tools/build.py): a vector, a fixture or a broken package in `fixtures/invalid/`.
+   Change the oracle in [src/paperspec/](src/paperspec/) to apply the rule, and add the case that shows it to [tools/build.py](tools/build.py): a vector, a fixture or a broken package in `fixtures/invalid/`.
    Then run `pixi run build`, which rewrites `fixtures/` and `vectors/`.
 4. **Run the checks.**
    `pixi run check` must end with `All checks pass.`
@@ -29,7 +29,7 @@ A rebuild that changes any file under `vectors/` or any hash in `fixtures/` is a
 ## New node kinds and run kinds
 
 Propose a kind with the vector or fixture that shows it.
-A node kind needs its row in [chapter 3](spec/03-nodes.md), the node schema, and its entry in the `CHILDREN` table of `tools/oracle/model.py`.
+A node kind needs its row in [chapter 3](spec/03-nodes.md), the node schema, and its entry in the `CHILDREN` table of `src/paperspec/model.py`.
 A run kind needs its row in [section 3.5](spec/03-nodes.md#35-inline-runs) and the node schema, and its text in the plain projection ([3.6](spec/03-nodes.md#36-the-plain-projection)) and in the strict and lossy projections ([5.3](spec/05-hashes.md#53-the-strict-projection), [5.5](spec/05-hashes.md#55-the-lossy-projection)).
 
 ## Releasing