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/walkthrough.md

@@ -13,7 +13,7 @@ It has nothing to do with the repository's name.
 
 Open [fixtures/packages/hale2026harmonic-damb/](../fixtures/packages/hale2026harmonic-damb/) beside this page, because every ID below is in it.
 A package is a folder of files, mostly JSON Lines (`.jsonl`: one JSON record per line).
-The hand-written source of the first reading is [fixtures/src/hale2026harmonic.json](../fixtures/src/hale2026harmonic.json); no converter ran on it, and [`tools/build.py`](../tools/build.py) expands it into the package.
+The hand-written source of the first reading is [fixtures/src/hale2026harmonic.json](../fixtures/src/hale2026harmonic.json); no converter ran on it, and [`paperspec-build`](../src/paperspec/cli/build.py) expands it into the package.
 Each step says what a converter, a person or a checker does.
 
 ## 1. A PDF arrives
@@ -166,7 +166,7 @@ It is a [Frictionless Data Package](https://datapackage.org/standard/data-packag
 Its CSV keeps every digit as printed (`1.526` and `1.5260` stay different), and every value has an anchor to the cell that prints it, so a claim that rests on a value can show the cell.
 The descriptor types the fields as numbers; how a generic Data Package tool treats `1.5260` was not verified.
 All twelve value anchors resolve as `current`.
-The reference checker, [`tools/check.py`](../tools/check.py), confirms each CSV value equals its cell's text.
+The reference checker, [`paperspec-check`](../src/paperspec/cli/check.py), confirms each CSV value equals its cell's text.
 
 ## 8. Sharing without the text