feat!: require Python 3.14 for the paperspec package

The specification pins the Unicode 16.0 character data (spec/01 §1.4),
which Python 3.14 is the first to ship; Python 3.12 has 15.0 and 3.13 has
15.1, so text with characters added in 16.0 could normalize differently
there. The tools already required 3.14; the package now does too.

- pyproject.toml: requires-python >= 3.14, with the 3.14 classifier.
- The package and tools docstrings, tools/README.md and the changelog say
  3.14 instead of 3.12.

pixi.lock still needs `pixi lock` (see the previous commit); its paperspec
entry also records the old requires-python.

pixi run check passes; no computed value changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -26,6 +26,6 @@ Packaging and lazy imports only; the formats stay at 0.1.0, and no computed valu
 
 - 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 base package has no dependencies and requires Python 3.14, the first with the Unicode 16.0 character data the specification pins; 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 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`.