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>

deploy/README.md

@@ -1,7 +1,7 @@
 # Deploying the website
 
 This folder holds what it takes to serve the paperspec website at <https://paperspec.scient.ing>.
-The site is static: `tools/site.py` builds it into `_site/`, and any web server can serve that folder.
+The site is static: `paperspec-site` builds it into `_site/`, and any web server can serve that folder.
 
 ## What the site serves
 
@@ -21,8 +21,8 @@ The pages need no server-side code; search runs in the browser over a small inde
 ## Build it
 
 ```sh
-pixi run python tools/site.py           # writes _site/
-pixi run python tools/site.py --out /tmp/paperspec-site
+pixi run site           # writes _site/
+pixi run site --out /tmp/paperspec-site
 ```
 
 `pixi run check` builds the site into a temporary folder and checks that every Markdown file has a page, that every internal link and fragment resolves, that no Markdown is left unrendered, and that each schema's `$id`, the context's address and the vocabulary's IRIs are files of the site.