harden pymol-open-source install check: import _cmd, not just launcher

Launcher existed while compiled ext was missing (partial compile ->
pure-python wheel installed from poisoned uv cache). Import pymol._cmd
at install time so a broken build dies in modbuild, not at runtime.

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

pymol-open-source/build

@@ -44,4 +44,8 @@ pbuild::install() {
     "$UV" pip install --python "$PREFIX/venv/bin/python" PyQt5 || \
         std::die 42 "PyQt5 install failed"
     [[ -x "$PREFIX/venv/bin/pymol" ]] || std::die 42 "pymol launcher missing from venv"
+    # launcher can exist while the compiled ext is missing (pure-python wheel from a
+    # poisoned uv cache); import _cmd directly so a broken build dies here, not at runtime
+    "$PREFIX/venv/bin/python" -c "import pymol._cmd" || \
+        std::die 42 "pymol._cmd import failed - ext .so missing; try 'uv cache clean pymol-open-source' and rebuild"
 }