ENH: delegate broadcast_shapes

[Cyril-36]

Part of #100.\n\nSummary:\n- Move the public broadcast_shapes wrapper into the delegation layer.\n- Delegate known integer shapes to numpy.broadcast_shapes when NumPy is available.\n- Preserve the existing fallback for None, math.nan, and no-NumPy runtimes.\n\nTesting:\n- pixi run -e tests pytest -q tests/test_funcs.py::TestBroadcastShapes\n- pixi run -e tests tests\n- pixi run -e lint ruff check src/array_api_extra/_delegation.py src/array_api_extra/_lib/_funcs.py src/array_api_extra/init.py tests/test_funcs.py\n- pixi run -e lint ruff format --check src/array_api_extra/_delegation.py src/array_api_extra/_lib/_funcs.py src/array_api_extra/init.py tests/test_funcs.py\n- pixi run -e lint mypy src/array_api_extra/_delegation.py src/array_api_extra/_lib/_funcs.py tests/test_funcs.py\n- pixi run -e lint pyright src/array_api_extra/_delegation.py src/array_api_extra/_lib/_funcs.py tests/test_funcs.py\n- pixi run -e lint pyrefly check src/array_api_extra/_delegation.py src/array_api_extra/_lib/_funcs.py tests/test_funcs.py\n- pixi run -e docs docs

[Cyril-36]

Updated based on Copilot review:\n- Added the numpydoc ignore marker to the internal fallback implementation.\n- Moved the optional NumPy import into a helper so NumPy is not imported at module import time.

[Cyril-36]

Thanks for the review. Updated the implementation to follow the existing delegation pattern: broadcast_shapes now accepts xp, delegates through xp.broadcast_shapes for supported namespaces and known integer shapes, and falls back to the generic implementation for unknown sizes or when no namespace is provided.

[Cyril-36]

Hi @lucascolley, just following up. I pushed the requested updates: broadcast_shapes now accepts xp, Dask was removed from the native delegation path, and the previous review comments appear outdated. Please let me know if anything else needs to be changed.

[Cyril-36]

Thanks, removed the redundant unknown-size fallback test.

[lucascolley]

thanks @Cyril-36 !