Convert unit tests into generated doctests

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We have a great deal of handwritten and LLM-generated unit tests, but the test coverage is somewhat spotty: they are spread across at least 3 different files and are rather disorganized, so the coverage is quite difficult to track.

At the same time the SIMD operations have no usage examples, just brief doc string descriptions.

We might be able to slay two birds with one stone by converting the test suite into doctests on the relevant methods. This moves the tests onto the methods themselves and makes coverage far easier to track, and has them double as usage examples in documentation.

Start by locating the handwritten and generated unit tests across the three or more test files, then identify the SIMD operation methods whose doc strings lack examples. Convert relevant coverage into doctests attached to those methods, preserving the existing test behavior and adding usage examples. Done means the tests run as doctests and coverage is easier to track from the methods themselves.

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