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.

tests/test_inc3_io.py

@@ -4,10 +4,9 @@ Round-trips rather than digests: STEP and IGES payloads carry timestamps and a
 generator banner, so byte comparison is meaningless — what has to survive is
 the geometry, which these check by measuring the shape that comes back.
 
-This file also settles open question **S5** from docs/design.md: whether the
-wheel has to ship OCCT's share/ resources for the XSTEP layer to resolve units
-when no CSF_* variable is set.  These tests run in exactly that situation — a
-container with the wheel and nothing else — so a pass means the answer is no.
+It also pins that the wheel needs none of OCCT's share/ resources for the
+XSTEP layer to resolve units when no CSF_* variable is set.  These tests run in
+exactly that situation, a container with the wheel and nothing else.
 """
 
 from __future__ import annotations
@@ -36,13 +35,13 @@ BOX = (10.0, 20.0, 30.0)
 
 @pytest.fixture(scope="module")
 def initialized():
-    """The app's own import bootstrap, and the S5 check in one.
+    """The app's own import bootstrap, and the resource-free check in one.
 
     If the controllers could not initialise without OCCT's resource files, this
     is where it would fail — there is no CSF_* in this environment.
     """
     assert not any(k.startswith("CSF_") for k in os.environ), (
-        "a CSF_* variable is set, so this run cannot answer S5"
+        "a CSF_* variable is set, so this run cannot prove the wheel needs none"
     )
     assert STEPControl_Controller.Init_s()
     assert IGESControl_Controller.Init_s()
@@ -112,7 +111,7 @@ def test_step_reports_its_file_units(initialized, tmp_path):
         TColStd_SequenceOfAsciiString(),
     )
     reader.FileUnits(length, angle, solid_angle)
-    assert length.Length() >= 1, "no length unit reported — S5 would be live"
+    assert length.Length() >= 1, "no length unit reported — the wheel would need OCCT's share/ resources"
     names = [length.Value(i).ToCString() for i in range(1, length.Length() + 1)]
     assert any("MM" in n.upper() or "MILLI" in n.upper() for n in names), names