feat: add ndarray/base/rot90

lib/node_modules/@stdlib/ndarray/base/rot90/README.md

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+<!--
+
+@license Apache-2.0
+
+Copyright (c) 2026 The Stdlib Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+-->
+
+# rot90
+
+> Rotate a matrix (or a stack of matrices) by 90 degrees.
+
+<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<!-- Package usage documentation. -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var rot90 = require( '@stdlib/ndarray/base/rot90' );
+```
+
+#### rot90( x, k, writable )
+
+Rotates a matrix (or a stack of matrices) `x` by 90 degrees.
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+
+var x = array( [ [ 1, 2 ], [ 3, 4 ] ] );
+// returns <ndarray>[ [ 1, 2 ], [ 3, 4 ] ]
+
+var y = rot90( x, 1, false );
+// returns <ndarray>[ [ 2, 4 ], [ 1, 3 ] ]
+```
+
+The function accepts the following arguments:
+
+-   **x**: input ndarray.
+-   **k**: number of times to rotate by 90 degrees. Positive values rotate counterclockwise. Negative values rotate clockwise.
+-   **writable**: boolean indicating whether a returned ndarray should be writable.
+
+</section>
+
+<!-- /.usage -->
+
+<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="notes">
+
+## Notes
+
+-   The `writable` parameter **only** applies to ndarray constructors supporting **read-only** instances.
+-   The returned ndarray is a **view** of the input ndarray. Accordingly, writing to the original ndarray will **mutate** the returned ndarray and vice versa.
+-   If provided an ndarray with fewer than two dimensions, the function returns the input array unchanged.
+
+</section>
+
+<!-- /.notes -->
+
+<!-- Package usage examples. -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var array = require( '@stdlib/ndarray/array' );
+var ndarray2array = require( '@stdlib/ndarray/to-array' );
+var rot90 = require( '@stdlib/ndarray/base/rot90' );
+
+// Create a 2x3 matrix:
+var x = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ] );
+
+// Rotate 90 degrees counterclockwise:
+var y = rot90( x, 1, false );
+var arr = ndarray2array( y );
+// returns [ [ 3, 6 ], [ 2, 5 ], [ 1, 4 ] ]
+
+// Rotate 180 degrees:
+y = rot90( x, 2, false );
+arr = ndarray2array( y );
+// returns [ [ 6, 5, 4 ], [ 3, 2, 1 ] ]
+
+// Rotate 270 degrees counterclockwise (equivalent to 90 degrees clockwise):
+y = rot90( x, 3, false );
+arr = ndarray2array( y );
+// returns [ [ 4, 1 ], [ 5, 2 ], [ 6, 3 ] ]
+
+// Rotate 360 degrees (equivalent to no rotation):
+y = rot90( x, 4, false );
+arr = ndarray2array( y );
+// returns [ [ 1, 2, 3 ], [ 4, 5, 6 ] ]
+
+// Rotate 90 degrees clockwise (equivalent to k=-1):
+y = rot90( x, -1, false );
+arr = ndarray2array( y );
+// returns [ [ 4, 1 ], [ 5, 2 ], [ 6, 3 ] ]
+```
+
+</section>
+
+<!-- /.examples -->
+
+<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="references">
+
+</section>
+
+<!-- /.references -->
+
+<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
+
+<section class="related">
+
+</section>
+
+<!-- /.related -->
+
+<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="links">
+
+<!-- <related-links> -->
+
+<!-- </related-links> -->
+
+</section>
+
+<!-- /.links -->