feat!: pin Python 3.14 and Unicode 16.0, and make the tools commands

Python and Unicode
- requires-python is >=3.14,<3.15: 3.14 is the only Python with the
  Unicode 16.0 data the specification pins (3.13 has 15.1, 3.15 has 17.0).
- `import paperspec` raises ImportError unless unicodedata reports
  paperspec.UNICODE_VERSION (16.0.0), so a source checkout or an install
  that bypasses requires-python cannot compute values on other data.

Commands
- tools/{check,build,validate,load,stress,site}.py move with history to
  src/paperspec/cli/ and install as paperspec-check, -build, -validate,
  -load, -stress and -site; each also runs as `python -m paperspec.cli.X`.
- paperspec.cli.require() exits with "install paperspec[tools]" when
  cryptography or jsonschema is missing; paperspec.cli.checkout() finds
  the repository the check, build, stress and site commands work on (the
  editable install's own checkout, else the one around the current
  folder) and exits with a message outside one. tools/__init__.py is gone.
- validate and load read the schemas and SQL through the new
  paperspec.resources.path(), which finds them in a wheel and in an
  editable install alike, so they work anywhere the package is installed.
- check runs the rebuild as `python -m paperspec.cli.build`.
- build still writes the provenance tool label "paperspec tools/build.py":
  it is part of every hashed and signed fixture.
- pixi tasks and the check workflow run the commands; tools/README.md
  documents them, and the other docs name the commands instead of paths.

License
- LICENSE.md gives `src/` Apache-2.0 alongside `tools/`. Since the
  previous commit moved the code to src/, no row had covered it.

pixi run check passes (run as `python -m paperspec.cli.check` with src/ on
the path, since this environment cannot install the entry points); the
byte-for-byte rebuild matches, so no fixture or vector changed.
pixi.lock still needs `pixi lock`.

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

LICENSE.md

@@ -4,7 +4,7 @@
 | --- | --- | --- |
 | `spec/`, `docs/`, `README.md`, `CONTRIBUTING.md`, `CHANGELOG.md` and the other prose | Creative Commons Attribution 4.0 International | `CC-BY-4.0` |
 | `schemas/`, `sql/`, `context/`, `registry/`, `vectors/`, `fixtures/` | Creative Commons Zero 1.0 Universal | `CC0-1.0` |
-| `tools/` | Apache License 2.0 | `Apache-2.0` |
+| `src/` (the `paperspec` package and its commands), `tools/` | Apache License 2.0 | `Apache-2.0` |
 
 The legal texts are at <https://creativecommons.org/licenses/by/4.0/legalcode>, <https://creativecommons.org/publicdomain/zero/1.0/legalcode> and <https://www.apache.org/licenses/LICENSE-2.0>.