Add furo to dev dependency group

alabaster rode along as a transitive dependency of sphinx itself, so
it never needed to be listed explicitly. furo has no such free ride,
so CI's `uv run sphinx-build` couldn't find the theme after the switch.

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

Author: derek73

pyproject.toml

@@ -42,6 +42,7 @@ dev = [
   "pytest (>=8)",
   "dill (>=0.2.5)",
   "sphinx (>=8)",
+  "furo",
   "mypy (>=2.1)",
   "ruff (>=0.15)",
   "pytest-timeout>=2.4.0",