test(notify): actually exercise rowstride, 3-channel and short-data paths

The stride test was Height 1, so the second row and the padding beyond the
used pixels were never reached and rowstride was untested despite the
comment. It is now 2x2 with RowStride 12 and asserts a pixel in row 1,
which fails if the stride is ignored (verified by mutation). Add the
3-channel opaque encode and the short-data rejection, both explicit
requirements of the image-data contract.

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