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>

tools/__init__.py

@@ -1,24 +0,0 @@
-"""The repository's own programs; see tools/README.md.
-
-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 needs Python 3.14 but neither package, and other programs install it on its own.
-"""
-
-import importlib.util
-import os
-import sys
-
-if sys.version_info < (3, 14):
-    sys.exit("The paperspec tools need Python 3.14 or later; this is Python %d.%d." % sys.version_info[:2])
-
-_missing = [m for m in ("cryptography", "jsonschema") if importlib.util.find_spec(m) is None]
-if _missing:
-    sys.exit("The paperspec tools need %s %s; run them through pixi, or install %s with pip."
-             % ("the package" if len(_missing) == 1 else "the packages", " and ".join(_missing),
-                "it" if len(_missing) == 1 else "them"))
-
-SRC = os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "src")
-if SRC not in sys.path:
-    sys.path.insert(0, SRC)