Pin sdist-install step to py3.13

The `sdist` CI job's install step ran bare `python -m pip
install`, picking up the runner's default py3.12 — which our
bumped `requires-python = ">=3.13"` now rejects
(`3.12.3 not in '<3.15,>=3.13'`).

- install into a `uv venv --python 3.13` so it matches the
  build step's `--python=3.13`.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

.github/workflows/ci.yml

@@ -37,7 +37,12 @@ jobs:
         run: uv build --sdist --python=3.13
 
       - name: Install sdist from .tar.gz
-        run: python -m pip install dist/*.tar.gz
+        # XXX must install under py3.13 (matching the build's
+        # `--python=3.13`); the runner's default `python` is 3.12
+        # which our `requires-python = ">=3.13"` now rejects.
+        run: |
+          uv venv --python 3.13
+          uv pip install dist/*.tar.gz
 
   # ------ type-check ------
   # mypy: