Own bulk-API ndarray buffers with std::vector, pin nanobind below 3

face_grid leaked both buffers when the kernel threw mid-grid, and tess copied
every mesh array a second time. Also drop the stale S5 references.

scripts/_env.sh

@@ -24,7 +24,10 @@ if [ ! -x "$VENV/bin/python" ]; then
     "$VENV/bin/pip" install -q --upgrade pip
     # numpy is a test dependency only: the n3xd_ocp bulk APIs hand back
     # ndarrays, so their tests need it to check the results.
-    "$VENV/bin/pip" install -q build nanobind scikit-build-core pytest numpy
+    # nanobind 3 reworked the type-caster and rv_policy interfaces
+    # occt_handle.h builds on, so moving to it is a deliberate bump, not
+    # something a fresh cache picks up.
+    "$VENV/bin/pip" install -q build "nanobind<3" scikit-build-core pytest numpy
 fi
 PY="$VENV/bin/python"
 export PY VENV CACHE