n3xd_ocp.measure: every face's area and centroid in one call

BRepGProp.SurfaceProperties is the hottest kernel call the app makes — 94 % of
face_candidate_anchors, 0.99 s of 1.05 s for 690 faces — not because the kernel
is slow but because it is reached once per face from Python, so a rebuild pays
the round-trip hundreds of times per feature over a growing shape.

face_surface_props(shape, *, parallel=True, eps=None) runs the whole scan
C++-side with the GIL released, optionally over OSD_Parallel, and returns
(areas[F], centroids[F,3]) in MapShapes(FACE) order — the face identity the
topology layer already keys on, so a caller indexes straight into it. A face
OCCT cannot integrate reports zeros, matching what the app's own try/except
substitutes.

Tested against the same stock-recorded per-face reference the Inc 1 gate uses,
and against the one-call-per-face loop it replaces. Parallel and serial are
compared with array_equal rather than approx: the parallel path shares one
TopoDS_Shape across threads, so an exact match is the evidence that nothing
reachable from it gets mutated while measuring.

Backend adoption comes after the cutover, so this changes nothing yet.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DfriM8XUkn7uYf5Dwe2xo6

tests/conftest.py

@@ -49,3 +49,11 @@ def a_face(fixture_shapes):
     faces = TopTools_IndexedMapOfShape()
     TopExp.MapShapes_s(fixture_shapes["box_meshed"], TopAbs_FACE, faces)
     return faces.FindKey(1)
+
+
+@pytest.fixture(scope="session")
+def inc1_or_skip(manifest):
+    """The Inc 1 reference block, for tests outside test_inc1_modeling.py."""
+    if "inc1" not in manifest:
+        pytest.skip("manifest predates the Inc 1 reference block")
+    return manifest["inc1"]