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>

fixtures/README.md

@@ -35,7 +35,7 @@ A conforming reader accepts each one and recomputes every hash it contains.
 ## Records
 
 `records/` holds a valid example of every other format that is a JSON file of its own.
-Three formats have their example elsewhere: the stage requirements (`scienting-stages/1`) and the stamp rules (`scienting-stamps/1`) are in [registry/](../registry/README.md), and a bundle (`scienting-bundle/1`) is a SQLite file that `tools/load.py --bundle` builds from the packages.
+Three formats have their example elsewhere: the stage requirements (`scienting-stages/1`) and the stamp rules (`scienting-stamps/1`) are in [registry/](../registry/README.md), and a bundle (`scienting-bundle/1`) is a SQLite file that `paperspec-load --bundle` builds from the packages.
 Each record is a single file, or a folder where a format needs several files.
 
 | Files | What they are for |
@@ -109,4 +109,4 @@ Use them for nothing else.
 ## Sources
 
 `src/` holds the hand-written descriptions of the three main documents, as first read; see [src/README.md](src/README.md).
-Everything else here is generated by `python3 tools/build.py`, so change the sources or the builder, never the generated files.
+Everything else here is generated by `paperspec-build`, so change the sources or the builder, never the generated files.

fixtures/src/README.md

@@ -1,6 +1,6 @@
 # Fixture sources
 
-Each source describes a document as a converter first read it, mistakes included, in a shorthand that `tools/build.py` expands.
+Each source describes a document as a converter first read it, mistakes included, in a shorthand that `paperspec-build` expands.
 The build mints IDs, applies the listed corrections, lays out synthetic geometry, signs the listed receipts and writes the package.
 The shorthand is tooling, not a format.