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>

pyproject.toml

@@ -10,11 +10,13 @@ readme = "README.md"
 license = "Apache-2.0"
 license-files = ["LICENSE.md"]
 authors = [{ name = "Scientific Computing Studio", email = "maintainer@scient.ing" }]
-requires-python = ">=3.12"
+# Python 3.14 is the first with the Unicode 16.0 character data the specification pins (spec/01 §1.4).
+requires-python = ">=3.14"
 classifiers = [
     "Development Status :: 3 - Alpha",
     "Intended Audience :: Science/Research",
     "Programming Language :: Python :: 3 :: Only",
+    "Programming Language :: Python :: 3.14",
     "Topic :: Scientific/Engineering",
 ]
 # The base package is standard library only. `schema.py` needs jsonschema and