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/examples.md

@@ -19,10 +19,10 @@ This shows how to confirm that the repository's specification, fixtures and test
 pixi run check
 ```
 
-`pixi run check` lints the Markdown, then runs `python tools/check.py`, the repository's reference checker.
+`pixi run check` lints the Markdown, then runs `paperspec-check`, the repository's reference checker.
 Each section prints how many checks it ran, and the last line is `All checks pass.`
-`pixi run quick`, or `python3 tools/check.py --quick`, skips the slowest part, which rebuilds everything and compares it byte for byte.
-Without pixi, the Markdown lint is a separate step (`markdownlint-cli2`); `tools/check.py` does not run it.
+`pixi run quick`, or `paperspec-check --quick`, skips the slowest part, which rebuilds everything and compares it byte for byte.
+Without pixi, the Markdown lint is a separate step (`markdownlint-cli2`); `paperspec-check` does not run it.
 
 ## Recompute a hash by hand
 
@@ -146,7 +146,7 @@ This shows that the same packages can live in a database and be queried with SQL
 pixi run load --sqlite lab.db fixtures/packages/hale2026harmonic-damb fixtures/packages/okafor2025solvent-acth fixtures/packages/writer2027fixed-ynr7.v3
 ```
 
-Without pixi, run `python3 tools/load.py` with the same arguments.
+Without pixi, run `paperspec-load` with the same arguments.
 
 ```text
 hale2026harmonic-damb: 75 nodes
@@ -154,7 +154,7 @@ okafor2025solvent-acth: 15 nodes
 writer2027fixed-ynr7.v3: 15 nodes
 ```
 
-The tables are those of [sql/doc.sql](../sql/doc.sql) and [sql/store.sql](../sql/store.sql), which run unchanged on Dolt and PostgreSQL as well ([chapter 13](../spec/13-database.md)); `tools/check.py` exercises them on SQLite only, and this repository has not yet run them on Dolt or PostgreSQL.
+The tables are those of [sql/doc.sql](../sql/doc.sql) and [sql/store.sql](../sql/store.sql), which run unchanged on Dolt and PostgreSQL as well ([chapter 13](../spec/13-database.md)); `paperspec-check` exercises them on SQLite only, and this repository has not yet run them on Dolt or PostgreSQL.
 Here are four queries, in Python's `sqlite3` so that nothing else is needed.
 
 ```python