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/overview.md

@@ -54,7 +54,7 @@ It appears inside hashed data and test vectors, and it is separate from the repo
 | [spec/](../spec/README.md) | The specification, in 24 chapters (00 to 23). Normative. |
 | [schemas/](../schemas/README.md) | 42 JSON (plain-text data format) Schemas for the files and records. |
 | [registry/](../registry/README.md) | The list of formats, the rule IDs a checker reports, download-stamp rules, and default rights requirements. |
-| [tools/](../tools/README.md) | Reference programs: `check.py` checks everything, `build.py` rebuilds the fixtures and vectors, and `load.py` loads packages into SQLite. |
+| [tools/](../tools/README.md) | How to run the reference commands: `paperspec-check` checks everything, `paperspec-build` rebuilds the fixtures and vectors, and `paperspec-load` loads packages into SQLite. |
 | [src/paperspec/](../src/paperspec/) | The oracle, a compact implementation of every rule, which the other repositories of the project install as the Python package `paperspec`. |
 | [vectors/](../vectors/README.md) | 25 files of test vectors: inputs and the exact outputs every implementation must compute. |
 | [fixtures/](../fixtures/README.md) | Five valid example packages, one of them also as a zip, 31 deliberately broken ones that each break one named rule, example records, and the test keys that signed them. All text is synthetic. |
@@ -73,8 +73,8 @@ The repository ships reference tools only for the checker and store roles.
 | Reader | Opens a package, resolves addresses and anchors, and shows text, annotations and each passage's fidelity level. |
 | Writer | Produces files in these formats: nodes, corrections, receipts, annotations, datasets. |
 | Converter | Turns a witness (a PDF, JATS XML, the article format publishers use, a LaTeX source archive, an HTML page, an EPUB or a Typst project) into a package and raises doubtful readings as suspects (flagged disagreements awaiting review). |
-| Checker | Recomputes hashes, validates files against their schemas and reports broken rules by rule ID. `tools/check.py` does this for the repository. |
-| Store | Keeps packages and records in a database a team can clone. `tools/load.py` loads packages into SQLite. |
+| Checker | Recomputes hashes, validates files against their schemas and reports broken rules by rule ID. `paperspec-check` does this for the repository. |
+| Store | Keeps packages and records in a database a team can clone. `paperspec-load` loads packages into SQLite. |
 
 The [specification's reading paths](../spec/README.md) say which chapters each role needs.