Rebuild stale `uv` envs after flake updates

`uv sync` accepts an existing same-minor Python even when its Nix
store path belongs to an older closure. That can leave a glibc 2.40
interpreter loading Qt built against glibc 2.42.

- derive the Python package once from `cpython`
- clear `py313` when its interpreter target differs from the flake
- ignore inherited `VIRTUAL_ENV` before syncing

(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))

flake.nix

@@ -29,6 +29,7 @@
           # XXX NOTE XXX, for now we overlay specific pkgs via
           # a major-version-pinned-`cpython`
           cpython = "python313";
+          python = pkgs.${cpython};
           pypkgs = pkgs."${cpython}Packages";
         in
         {
@@ -47,7 +48,7 @@
               qt6.qtbase
 
               uv
-              python313  # ?TODO^ how to set from `cpython` above?
+              python
               pypkgs.pyqt6
               pypkgs.pyqt6-sip
               pypkgs.qtpy
@@ -88,8 +89,23 @@
               export QT_WAYLAND_SHELL_INTEGRATION="xdg-shell"
 
               # ------ uv ------
+              # ignore any venv inherited from an outer checkout
+              unset VIRTUAL_ENV
               # - always use the ./py313/ venv-subdir
               export UV_PROJECT_ENVIRONMENT="py313"
+
+              # Rebuild after a flake update changes the Nix Python closure.
+              venv_python="$UV_PROJECT_ENVIRONMENT/bin/python"
+              nix_python="$(readlink -f "${python}/bin/python3")"
+              if test -e "$venv_python" \
+                && test "$(readlink -f "$venv_python")" != "$nix_python"
+              then
+                uv venv \
+                  --clear \
+                  --python "$nix_python" \
+                  "$UV_PROJECT_ENVIRONMENT"
+              fi
+
               # sync project-env with all extras
               uv sync --dev --all-extras --no-group lint
               source "$UV_PROJECT_ENVIRONMENT/bin/activate"