Fix `flake.nix` dev-shell install

The refreshed Nix closure no longer supplies `libz.so.1` to the
`uv` NumPy wheel, and leaves the synced `py313` env off `PATH`.

- add Nix `zlib` to `LD_LIBRARY_PATH` for native Python deps
- activate `py313` after `uv sync` so `pyqtgraph`, `piker` and
  `xonsh` resolve from the locked env

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

flake.nix

@@ -22,6 +22,7 @@
 
           # do store-path extractions
           qt6baseStorePath = lib.getLib pkgs.qt6.qtbase;
+          zlibStorePath = lib.getLib pkgs.zlib;
           # ?TODO? can remove below since manual linking not needed?
           # qt6QtWaylandStorePath = lib.getLib pkgs.qt6.qtwayland;
 
@@ -70,6 +71,7 @@
 
               # link-in c++ stdlib for various AOT-ext-pkgs (numpy, etc.)
               LD_LIBRARY_PATH="${pkgs.stdenv.cc.cc.lib}/lib:$LD_LIBRARY_PATH"
+              LD_LIBRARY_PATH="${zlibStorePath}/lib:$LD_LIBRARY_PATH"
 
               export LD_LIBRARY_PATH
 
@@ -90,11 +92,12 @@
               export UV_PROJECT_ENVIRONMENT="py313"
               # sync project-env with all extras
               uv sync --dev --all-extras --no-group lint
+              source "$UV_PROJECT_ENVIRONMENT/bin/activate"
 
               # ------ TIPS ------
               # NOTE, to launch the py-venv installed `xonsh` (like @goodboy)
               # run the `nix develop` cmd with,
-              # >> nix develop -c uv run xonsh
+              # >> nix develop -c xonsh
             '';
           };
         }