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/faq.md

@@ -107,7 +107,7 @@ A `candidate` format is complete, with vectors and fixtures, and ready to implem
 A `provisional` one is complete and expected to change as implementations gain experience.
 The provisional formats are dataset, digitization, view, project, bundle, snapshot, draft-ids, exercise, submission, grade, checklist, log-entry and checkpoint.
 
-The repository holds a reference checker (`tools/check.py`), 25 vector files, five valid packages, one of them also as a zip, and 31 deliberately broken packages ([spec/00 §0.4](../spec/00-introduction.md#04-status), [spec/22](../spec/22-conformance.md)).
+The repository holds a reference checker (`paperspec-check`), 25 vector files, five valid packages, one of them also as a zip, and 31 deliberately broken packages ([spec/00 §0.4](../spec/00-introduction.md#04-status), [spec/22](../spec/22-conformance.md)).
 
 ## What does it not do?
 
@@ -126,6 +126,6 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md).
 
 ## What are the files in `tools/` for?
 
-`build.py` builds the fixtures and vectors. `check.py` recomputes and checks every one, and `load.py` loads packages into SQLite.
+`paperspec-build` builds the fixtures and vectors. `paperspec-check` recomputes and checks every one, and `paperspec-load` loads packages into SQLite.
 The oracle's modules in `src/paperspec/` compute every expected value, so that none is typed by hand.
 The other repositories of the project install them as the Python package `paperspec`; independent implementations follow the specification ([tools/README.md](../tools/README.md)).