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>

docs/comparison.md

@@ -428,7 +428,7 @@ Keep using it as a reading and distribution format.
 *Status.*
 The specification is version 0.1.0.
 `registry/formats.json` lists 32 formats: 19 are `candidate` (complete, with vectors and fixtures, ready to implement) and 13 are `provisional` (complete; expected to change as implementations gain experience): project, bundle, dataset, digitization, view, snapshot, draft-ids, exercise, submission, grade, checklist, log-entry and checkpoint ([spec/00 §0.4](../spec/00-introduction.md#04-status)).
-The repository holds 24 chapters, 42 JSON Schemas, 25 vector files, five valid fixture packages, one of them also zipped, and 31 invalid ones, each naming the rule it breaks, and a reference checker, `tools/check.py`.
+The repository holds 24 chapters, 42 JSON Schemas, 25 vector files, five valid fixture packages, one of them also zipped, and 31 invalid ones, each naming the rule it breaks, and a reference checker, `paperspec-check`.
 Licenses: prose CC BY 4.0; schemas, SQL, the JSON-LD context, registries, vectors and fixtures CC0 1.0; tools Apache-2.0 ([README.md](../README.md#licenses)).
 
 *No adoption claim.*