feat: install tools/oracle as the paperspec package

Other repositories (paperstack, then paperpack) import the reference
implementation instead of reimplementing the specification; scienting-core
will take the role later behind the same vectors.

- pyproject.toml: distribution and import name `paperspec`, version 0.1.0,
  Python >= 3.12, no base dependencies; `tools/oracle` is the package, and
  `schemas/`, `sql/` and `vectors/` ship as the data subpackages
  `paperspec.schemas`, `paperspec.sql` and `paperspec.vectors`. Extras
  `schema`, `crypto` and `tools`.
- tools/oracle/__init__.py: the Python 3.14 and cryptography/jsonschema gate
  runs only when imported as `tools.oracle` by the repository's own tools.
- crypto.py imports `cryptography` on first use of HKDF and Ed25519, with an
  ImportError naming `paperspec[crypto]`; schema.py imports `jsonschema` and
  `referencing` in `Store.validator` and `Store.registry`. package.py, db.py
  and receipts.py then import on a bare interpreter.
- pixi.toml installs the package editable (pixi.lock re-solved; no conda pin
  changed), so the tools and the package resolve to the same code.
- tools/README.md names the library surface and the extras; CHANGELOG.md
  records the packaging change under 0.1.0, since check.py requires the
  changelog's first heading to equal the format version.

No computed value, vector, fixture, schema or SQL file changed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tools/oracle/__init__.py

@@ -1,13 +1,21 @@
-"""The reference computations behind every expected value in this repository; see tools/README.md."""
+"""The reference computations behind every expected value in this repository; see tools/README.md.
+
+Imported as `tools.oracle` by the repository's own tools, which need Python 3.14, `cryptography` and
+`jsonschema` for everything they check; the gate below exits with a message when one is missing.
+Installed and imported as `paperspec` by other programs, the gate is skipped: the standard-library modules
+(`keys`, `text`, `jcs`, `package`, `db`, `rights`, ...) run on Python 3.12, and `schema` and `crypto` import
+their packages on first use, so `paperspec[schema]` and `paperspec[crypto]` are needed only for those two.
+"""
 
 import importlib.util
 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])
+if __name__ == "tools.oracle":
+    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"))
+    _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"))