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>

tools/__init__.py

@@ -3,7 +3,7 @@
 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.
+itself needs Python 3.14 but neither package, and other programs install it on its own.
 """
 
 import importlib.util