What

Adds a new public method ArrayBase::rot90(k, axes) that rotates an array by 90 degrees k times in the plane spanned by two axes, mirroring NumPy's numpy.rot90.

• Rotation goes from axes.0 toward axes.1 (counterclockwise in that axis ordering), matching NumPy.
• k is reduced modulo 4 via rem_euclid, so negative k rotates in the opposite (clockwise) direction and k == -1 equals k == 3.
• No data is copied: it only adjusts dimensions and strides via the existing swap_axes / invert_axis, so it is O(1). It consumes self; call it on .view()/.clone() to keep the original.
• #[track_caller], and panics if the two axes are equal or out of bounds.

Why

There is currently no built-in equivalent to np.rot90; users have to hand-compose swap_axes/invert_axis (or multiply by a flipped identity matrix). Issue #866 (help-wanted) requests it, and a maintainer asked for a volunteer to implement the full NumPy-equivalent including the k and axes parameters.

Testing

• Added test_rot90 in tests/array.rs covering all four rotation cases against hand-verified NumPy results on arr2(&[[1,2,3],[4,5,6]]), the k == 0 / k == 4 identity, negative-k (clockwise) behavior, reversed-axis-plane direction, and a 3-D shape check ((1,2,3) rotated in plane (0,2) -> [3,2,1]).
• Added test_rot90_same_axis and test_rot90_oob #[should_panic] tests for the two panic conditions.
• Included a rustdoc doc-test on the method.
• cargo test --test array test_rot90 -> 3 passed; cargo test --doc -- rot90 -> doc-test passes.
• Added an entry to RELEASES.md.

Fixes #866.