ponder-lab / Hybridize-Functions-Refactoring / #3929

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Refuse a function that returns a NumPy array to a caller (#1200)

* Refuse a function that returns a NumPy array to a caller

A function that only slices and combines an array it is passed returns an
array eagerly, but under `tf.function` the array crosses the boundary as a
tensor and the function returns a tensor, so a caller using the result as an
array breaks. The new RETURNS_NUMPY_ARRAY precondition reads the engine's
origin of each call site's result and refuses when it contains NumPy, which
includes a result that is an array on one path and a tensor on another.

Closes #1199.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NTnpjPjJj2aZy1ZkUD1eAn

* Test that a function returning its NumPy argument unchanged is refused

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NTnpjPjJj2aZy1ZkUD1eAn

* Say why the NumPy-return check's defensive branches are untested

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NTnpjPjJj2aZy1ZkUD1eAn

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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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