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.

scripts/_env.sh

@@ -0,0 +1,28 @@
+# Shared environment for the in-container scripts.  Sourced, not executed.
+#
+# Everything expensive or bulky (ccache, the CMake tree, pip's cache, the build
+# venv) lives under $CACHE, which the Makefile bind-mounts from outside the
+# repo — the root filesystem on this host has little headroom, and these
+# survive between runs to keep the inner loop incremental.
+
+CACHE="${CACHE_ROOT:-/cache}"
+export CCACHE_DIR="$CACHE/ccache"
+export PIP_CACHE_DIR="$CACHE/pip"
+export HOME="$CACHE"           # containers run as the host uid; /root is not ours
+export CMAKE_C_COMPILER_LAUNCHER=ccache
+export CMAKE_CXX_COMPILER_LAUNCHER=ccache
+export CMAKE_PREFIX_PATH=/opt/occt
+export LD_LIBRARY_PATH=/opt/occt/lib
+export SKBUILD_BUILD_DIR="$CACHE/skbuild/{wheel_tag}"
+
+mkdir -p "$CCACHE_DIR" "$PIP_CACHE_DIR"
+
+VENV="$CACHE/venv"
+if [ ! -x "$VENV/bin/python" ]; then
+    echo "--- creating build venv at $VENV ---"
+    /opt/python/cp312-cp312/bin/python -m venv "$VENV"
+    "$VENV/bin/pip" install -q --upgrade pip
+    "$VENV/bin/pip" install -q build nanobind scikit-build-core pytest
+fi
+PY="$VENV/bin/python"
+export PY VENV CACHE