Phase 10A/10B Inc 0: build system, handle model, first module surface

Builds n3xd-ocp end to end and publishes 7.9.3.1.dev1 to the Gitea registry,
where it installs anonymously and passes its suite.

- occt/Dockerfile: OCCT 7.9.3 compiled once into a manylinux_2_28 builder
  image (base digest + tarball sha256 pinned), Draw/VTK/Tk/Xlib/OpenGL off,
  FreeType on, -O2 without fast-math or march=native. A final layer asserts
  TKService/TKV3d exist with no libGL/libX11 DT_NEEDED, which is what lets the
  app image drop libgl1/libx11-6. Mounted into, never built FROM.
- scikit-build-core + nanobind STABLE_ABI -> one cp312-abi3 extension that
  registers every OCP.* submodule via PyImport_AddModule, so `import
  OCP.TopoDS` needs no shim and cls.__module__ is right. Version <occt>.N is
  asserted against the OCCT found, keeping occt_version() truthful.
- occt_handle.h: type caster for opencascade::handle<T> over OCCT's intrusive
  refcount. Wrappers are non-owning instances holding exactly one handle in
  their keep-alive list, reusing an existing wrapper so identity survives a
  round trip. Transient constructors go through ocp_new (never nb::init<>,
  which would let OCCT delete nanobind's storage); the caster refuses a
  refcount-0 object rather than corrupt the heap. Verified under ASAN with no
  memory-safety errors, plus an RSS bound over 50k create/destroy cycles.
- Sub-shapes are returned by value everywhere, making the TShape lifetime class
  that segfaulted a process-global face memo unrepresentable.
- Standard_Failure derives RuntimeError, with ~20 concrete types dispatched on
  the dynamic OCCT type (cad_pool marshals failures home by type name).
- Inc 0 surface: gp subset, TopAbs, TopoDS (+ downcasts), TopExp, TopLoc,
  TopTools, BRep, BinTools, Poly, Standard. 34 of the app's 139 symbols.
- n3xd_ocp: additive APIs kept out of the OCP namespace so parity testing stays
  meaningful. bintools (shape <-> bytes, GIL-free, byte-identical) and _debug.

Two findings worth the record, both verified against the stock wheel rather
than assumed: upstream binds __hash__ but leaves __eq__ at identity, which is
exactly what geom_memo.py's hash-bucket + IsSame scan is built around, so we
match it instead of "fixing" it; and BinTools can release the GIL after all, by
slurping the file-like object instead of bridging a streambuf that would call
back into Python.

Gate: BREP round-trips are byte-identical to cadquery-ocp-novtk across six
fixtures (the generator asserts stock idempotency first). That matters beyond
IPC — derive.py content-addresses BREP payloads by sha256 and stores the ref.

python/OCP/__init__.py

@@ -0,0 +1,25 @@
+"""OCP — Python bindings for the OpenCASCADE (OCCT) geometry kernel.
+
+Installs under the same import path as ``cadquery-ocp``/``cadquery-ocp-novtk``
+so it is a drop-in replacement; the two can never be installed together, and a
+process can only ever hold one OCCT build.
+
+The compiled extension registers every ``OCP.*`` submodule directly in
+``sys.modules``, so ``import OCP.TopoDS`` works without a shim module per name.
+This file only re-exports them as attributes, so ``OCP.TopoDS`` also resolves
+after a bare ``import OCP``.
+"""
+
+from __future__ import annotations
+
+import sys
+
+from . import _OCP
+
+__version__ = _OCP.__version__
+__occt_version__ = _OCP.__occt_version__
+
+for _name in _OCP.__all_modules__:
+    setattr(sys.modules[__name__], _name, sys.modules[f"OCP.{_name}"])
+
+__all__ = [*_OCP.__all_modules__, "__version__", "__occt_version__"]

python/n3xd_ocp/__init__.py

@@ -0,0 +1,21 @@
+"""n3xd_ocp — performance APIs that go beyond the upstream OCP surface.
+
+Kept separate from ``OCP`` on purpose: that namespace stays a symbol-for-symbol
+drop-in for ``cadquery-ocp-novtk`` so parity testing means something, while
+anything additive lives here. Shipped in the same wheel, backed by the same
+OCCT build, and safe to mix with ``OCP`` objects in one process.
+
+Submodules are registered by the compiled extension; importing ``OCP`` is what
+loads it.
+"""
+
+from __future__ import annotations
+
+import sys
+
+import OCP as _OCP_pkg  # noqa: F401  (loads the extension, registering us)
+
+bintools = sys.modules["n3xd_ocp.bintools"]
+_debug = sys.modules["n3xd_ocp._debug"]
+
+__all__ = ["bintools"]