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>

tools/__init__.py

@@ -0,0 +1,24 @@
+"""The repository's own programs; see tools/README.md.
+
+Importing this package checks that the tools can run here, and puts `src/` first on the path, so that
+`import paperspec` gets this checkout's reference implementation rather than an installed copy.
+The tools need Python 3.14 and the packages `cryptography` and `jsonschema`; the `paperspec` package
+itself does not, and other programs install it on its own.
+"""
+
+import importlib.util
+import os
+import sys
+
+if sys.version_info < (3, 14):
+    sys.exit("The paperspec tools need Python 3.14 or later; this is Python %d.%d." % sys.version_info[:2])
+
+_missing = [m for m in ("cryptography", "jsonschema") if importlib.util.find_spec(m) is None]
+if _missing:
+    sys.exit("The paperspec tools need %s %s; run them through pixi, or install %s with pip."
+             % ("the package" if len(_missing) == 1 else "the packages", " and ".join(_missing),
+                "it" if len(_missing) == 1 else "them"))
+
+SRC = os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "src")
+if SRC not in sys.path:
+    sys.path.insert(0, SRC)