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>

README.md

@@ -61,8 +61,8 @@ The address grammar is in [spec/06 §6.1](spec/06-anchors.md#61-addresses), and
 | [registry/](registry/README.md) | The list of formats, the rule IDs validators report, download-stamp rules, and the default rights each processing stage requires. |
 | [vectors/](vectors/README.md) | Test vectors: inputs and the exact outputs every implementation must compute. |
 | [fixtures/](fixtures/README.md) | Example packages and records, deliberately broken packages, and the test keys that signed them. |
-| [tools/](tools/README.md) | The programs that build and check everything above. |
-| [src/paperspec/](src/paperspec/) | The reference implementation of every rule, installable as the Python package `paperspec`. |
+| [tools/](tools/README.md) | How to run the commands that build and check everything above. |
+| [src/paperspec/](src/paperspec/) | The reference implementation of every rule and those commands, installable as the Python package `paperspec`. |
 | [docs/](docs/README.md) | Guides for people getting to know the project. |
 | [deploy/](deploy/README.md) | How the website at <https://paperspec.scient.ing> is built and served. |
 
@@ -81,10 +81,10 @@ Last, it loads every package into SQLite and back, and checks the prose and the
 Pixi lints the Markdown first.
 The run ends with `All checks pass.`
 `pixi task list` shows the other tasks: `quick`, `build`, `lint`, `load`, `validate` and `stress`.
-`pixi run python tools/site.py` builds the website into `_site/` ([deploy/](deploy/README.md)).
+`pixi run site` builds the website into `_site/` ([deploy/](deploy/README.md)).
 
 The tools need Python 3.14 and two packages, `jsonschema` and `cryptography`, which pixi installs along with markdownlint for the Markdown.
-Without pixi, install those two with pip and run `python3 tools/check.py`.
+Without pixi, install the checkout as the package with `pip install -e ".[tools]"` on Python 3.14, then run `paperspec-check`.
 
 To explore a worked package, load the example paper (a synthetic chemistry paper with 12 corrections) into SQLite and query it; [docs/examples.md](docs/examples.md) has queries to start from.
 
@@ -132,5 +132,5 @@ Table columns and figure-text items have IDs and are not nodes ([spec/04 §4.2](
 
 The specification and other prose are CC BY 4.0, so anyone may implement the formats.
 Schemas, SQL, the JSON-LD context, the registries, the vectors and the fixtures are CC0 1.0.
-The tools are Apache-2.0.
+The code, the `paperspec` package in `src/` with its commands, is Apache-2.0.
 See [LICENSE.md](LICENSE.md).