Scaffold the n3xd-ocp binding repo

Placeholder for the hand-written nanobind bindings over OCCT (roadmap Phase 10
in the app repo). Carries the gitignore for a scikit-build-core/CMake
extension plus the OCCT build trees, and keeps .secrets — the Gitea publish
PAT — out of history.

README.md

@@ -0,0 +1,47 @@
+# n3xd-ocp
+
+Hand-written [nanobind](https://github.com/wjakob/nanobind) bindings for the
+OpenCASCADE (OCCT) geometry kernel, covering exactly the surface the N3XD CAD
+backend uses — roughly 140 symbols across 48 `OCP.*` modules, not all of OCCT.
+
+The package installs as a top-level `OCP`, so it is a drop-in replacement for
+`cadquery-ocp-novtk` and the app's 437 import sites stay untouched.
+
+Status: **not started.** The plan lives in the app repo at
+`docs-private/reference/roadmap.md` (Phase 10), with the binding-layer
+background in `docs-private/architecture/geometry-kernel.md`.
+
+## Why
+
+`cadquery-ocp` lags OCCT (it wraps 7.9.3; OCCT 8.0 shipped 2026-05), builds
+Windows and macOS wheels we never use, and until recently forced a 638 MB VTK
+dependency into the image. Binding *call* overhead is not a bottleneck — the
+CAD hotspots live inside the C++ kernel — so this exists for version velocity,
+footprint, and two defects that a binding we control prevents by construction:
+
+- OCCT sub-shapes are returned **by value**, so a wrapper can never alias a
+  `TShape` whose owner has died (this segfaulted a process-global face memo).
+- Executing constructors (the two-argument `BRepAlgoAPI_*` forms) are **not
+  bound**, so the double-execution footgun is unrepresentable.
+
+It also releases the GIL around kernel calls and ships type stubs, neither of
+which upstream does.
+
+## Build
+
+OCCT is compiled once into a builder image and reused; it is never built on the
+production host (4 cores, and a kernel build is multi-hour). Wheels are built
+here on a dev box and published to the Gitea package registry:
+
+```bash
+uv publish --publish-url https://git.stroblme.de/api/packages/N3XD/pypi dist/*.whl
+```
+
+Credentials go in `.secrets` (gitignored) as `UV_PUBLISH_USERNAME` /
+`UV_PUBLISH_PASSWORD`. Consumers read anonymously — the package is public — via
+`https://git.stroblme.de/api/packages/N3XD/pypi/simple/`.
+
+Versions are `<occt-version>.N`, so the kernel a wheel wraps is readable from
+its version alone. The registry refuses to republish a version; iteration
+builds therefore carry a `.devN` suffix and are the only ones the registry's
+cleanup rule collects.