10C Inc 1: core modeling

Binds the modeling core the app builds every feature out of — 93 of its 138
symbols now resolve, up from 34. New modules: GeomAbs, Geom2d, Geom,
TCollection, TColgp, TColStd, GProp, BRepGProp, Bnd, BRepBndLib, Adaptor3d,
BRepAdaptor, GeomLProp, GeomAPI, BRepBuilderAPI, BOPAlgo, BRepAlgoAPI,
BRepPrimAPI, GC, BRepMesh; gp and BRep_Tool completed.

Three structural decisions:

- BRepBuilderAPI_MakeShape carries Build/Shape/Generated/Modified/IsDeleted for
  every maker in the binding, so the booleans, the primitives and (later) the
  fillet builders all answer the app's duck-typed provenance layer through
  ordinary virtual dispatch. History lists come back copied, so they outlive
  the builder.
- The executing two-argument BRepAlgoAPI constructors stay unbound; operands go
  in through SetArguments/SetTools. Section keeps Init1/Init2, which are plain
  setters. BOPAlgo moved up from Inc 2 — SetGlue needs its enum.
- Adaptor3d is registered although the app never imports it: every method it
  calls on BRepAdaptor_Curve/Surface is a virtual declared there, so binding
  them once on the bases leaves mod_BRepAdaptor.cpp with just constructors.

Gate: tests/test_inc1_modeling.py against reference values gen_fixtures.py now
records from the stock wheel — measurements, per-face area/centroid in map
order, mesh counts, and the boolean history map compared exactly, since that is
the substrate the app's topological naming is built on.

tools/sigdiff.py compares our bound surface against stock's, because a wrong
nb::init<> is silent: MakePrism's five-argument form bound OCCT's semi-infinite
gp_Dir overload (gp_Dir converts from gp_Vec), producing a valid solid of the
wrong shape with the flags shifted along. The fixture digest caught it; sigdiff
finds the class of bug directly, and now reports only one deliberate deviation.

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

src/modules/mod_GC.cpp

@@ -0,0 +1,25 @@
+/*
+    OCP.GC — 3D geometry constructors.
+
+    One use: the sketch builder turns a three-point arc into a trimmed circle
+    before handing it to BRepBuilderAPI_MakeEdge.
+*/
+
+#include "../common/occt_module.h"
+
+#include <GC_MakeArcOfCircle.hxx>
+#include <gp_Circ.hxx>
+#include <gp_Pnt.hxx>
+
+void register_GC(nb::module_ &root) {
+    nb::module_ m = ocp_submodule(root, "GC");
+
+    nb::class_<GC_MakeArcOfCircle>(m, "GC_MakeArcOfCircle")
+        .def(nb::init<const gp_Pnt &, const gp_Pnt &, const gp_Pnt &>(), "P1"_a,
+             "P2"_a, "P3"_a)
+        .def(nb::init<const gp_Circ &, const gp_Pnt &, const gp_Pnt &,
+                      const Standard_Boolean>(),
+             "Circ"_a, "P1"_a, "P2"_a, "Sense"_a)
+        .def("IsDone", &GC_MakeArcOfCircle::IsDone)
+        .def("Value", &GC_MakeArcOfCircle::Value);
+}