Pin setuptools>=83.0.0 as a dev dependency

The build-system requirement only pins the isolated build backend, not
the runtime venv setuptools that pip-audit scans, so CI still saw the
vulnerable 79.0.1. Add it to the dev extra so pip actually installs
the patched version.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pyproject.toml

@@ -49,6 +49,7 @@ dev = [
     "ruff>=0.6",
     "ty>=0.0.1",
     "pip-audit>=2.7",
+    "setuptools>=83.0.0",
 ]
 plotting = [
     "matplotlib>=3.7",