[codex] Add pointer-backed return materialization

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Summary

Adds an explicit pointer-backed return materialization path for APIs that return borrowed fixed-size pointer data, such as const float4* get_transform(handle). Configured functions now expose a Numba-visible UniTuple(pointee, length) return, with lowering copying the configured number of elements from the returned pointer.

Changes

• Added PointerReturnMaterialization and config parsing for dynamic and static binding paths.
• Extended FunctionCallConv to load fixed-size pointee data from the C++ return pointer and materialize it as a tuple return.
• Added static binding config support via Function Return Materializations, including schema and generated schema docs.
• Added dynamic/static tests and CUDA fixture declarations for a const float4* return materialized as UniTuple(float32x4, 3).
• Documented the new config shape in the argument intents guide.

Validation

• PYTHONPATH=/home/wangm/numbast-optix/numbast-pointer-backed-return/numbast/src:/home/wangm/numbast-optix/numbast-pointer-backed-return/ast_canopy pytest -q numbast/tests/test_callconv.py numbast/tests/test_function.py::test_pointer_return_binding_signature numbast/src/numbast/static/tests/test_function_static_bindings.py::test_pointer_return_static_binding_source numbast/src/numbast/static/tests/test_function_static_bindings.py::test_generated_callconv_alignof_helper_is_standalone numbast/src/numbast/tools/tests/test_config_schema_docs.py
• Result: 12 passed in 0.87s

CUDA execution tests were added but could not be run on this machine because CUDA initialization reports CUDA_ERROR_NO_DEVICE.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/pointer-backed-return

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