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/asan.sh

@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+# Memory-safety check for the handle model.
+#
+# ASAN's leak detector is not usable here — CPython's arenas and OCCT's
+# process-lifetime singletons produce noise that would drown a real finding —
+# so this run targets *safety*: use-after-free, double-free, buffer overflow.
+# That is exactly the failure mode a wrong ownership rule produces (a stale
+# TShape is what segfaulted a process-global face memo under upstream OCP).
+# Leak *growth* is checked separately, by the RSS assertion in test_handles.py.
+set -euo pipefail
+
+cd /io
+. scripts/_env.sh
+
+BUILD="$CACHE/asan"
+mkdir -p "$BUILD"
+
+echo "--- configuring ASAN build ---"
+cmake -G Ninja -S . -B "$BUILD" \
+    -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+    -DCMAKE_CXX_FLAGS="-fsanitize=address -fno-omit-frame-pointer -g" \
+    -DCMAKE_SHARED_LINKER_FLAGS="-fsanitize=address" \
+    -DPython_EXECUTABLE="$PY" \
+    -Dnanobind_DIR="$("$PY" -c 'import nanobind; print(nanobind.cmake_dir())')" \
+    >/dev/null
+
+cmake --build "$BUILD" -j"$(nproc)"
+
+STAGE=$(mktemp -d)
+trap 'rm -rf "$STAGE"' EXIT
+cp -r python/OCP python/n3xd_ocp tests "$STAGE/"
+cp "$BUILD"/_OCP*.so "$STAGE/OCP/"
+cp -r tests/data "$STAGE/tests/" 2>/dev/null || true
+
+echo "--- running handle tests under ASAN ---"
+cd "$STAGE"
+# libasan.so is a linker script, not an ELF object — preloading it is silently
+# ignored and the run would look clean while ASAN was never active.  Resolve
+# the real soname instead, and fail loudly if it is missing.
+ASAN_LIB=$(gcc -print-file-name=libasan.so.8)
+[ -f "$ASAN_LIB" ] || { echo "libasan.so.8 not found ($ASAN_LIB)" >&2; exit 1; }
+head -c 4 "$ASAN_LIB" | grep -q ELF || { echo "$ASAN_LIB is not an ELF object" >&2; exit 1; }
+
+LD_PRELOAD="$ASAN_LIB" \
+N3XD_OCP_ASAN=1 \
+ASAN_OPTIONS="detect_leaks=0:abort_on_error=1:strict_string_checks=1:detect_stack_use_after_return=1" \
+    "$PY" -m pytest tests/test_handles.py tests/test_shape_identity.py -q
+
+echo "ASAN: no memory-safety errors"