feat: add ndarray/base/fill-diagonal

lib/node_modules/@stdlib/ndarray/base/fill-diagonal/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.
+
+-->
+
+# fillDiagonal
+
+> Fill a specified diagonal of a matrix (or stack of matrices) with a scalar value.
+
+<section class="intro">
+
+For an `M`-by-`N` matrix `A`, the `k`-th diagonal is defined as
+
+<!-- <equation class="equation" label="eq:diagonal_definition" align="center" raw="D_k = \{\, A_{i,j} : j - i = k \,\}" alt="Definition of the k-th diagonal of a matrix."> -->
+
+```math
+D_k = \{\, A_{i,j} : j - i = k \,\}
+```
+
+<!-- <div class="equation" align="center" data-raw-text="D_k = \{\, A_{i,j} : j - i = k \,\}" data-equation="eq:diagonal_definition">
+    <img src="" alt="Definition of the k-th diagonal of a matrix.">
+    <br>
+</div> -->
+
+<!-- </equation> -->
+
+where `k = 0` corresponds to the main diagonal, `k > 0` corresponds to the super-diagonals (above the main diagonal), and `k < 0` corresponds to the sub-diagonals (below the main diagonal).
+
+</section>
+
+<!-- /.intro -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var fillDiagonal = require( '@stdlib/ndarray/base/fill-diagonal' );
+```
+
+#### fillDiagonal( x, value, dims, k )
+
+Fills a specified diagonal of a matrix (or stack of matrices) with a scalar value.
+
+```javascript
+var zeros = require( '@stdlib/ndarray/zeros' );
+
+var x = zeros( [ 3, 3 ] );
+// returns <ndarray>[ [ 0.0, 0.0, 0.0 ], [ 0.0, 0.0, 0.0 ], [ 0.0, 0.0, 0.0 ] ]
+
+var out = fillDiagonal( x, 1.0, [ 0, 1 ], 0 );
+// returns <ndarray>[ [ 1.0, 0.0, 0.0 ], [ 0.0, 1.0, 0.0 ], [ 0.0, 0.0, 1.0 ] ]
+
+var bool = ( out === x );
+// returns true
+```
+
+The function accepts the following arguments:
+
+-   **x**: input ndarray.
+-   **value**: scalar value.
+-   **dims**: dimension indices defining the plane in which to fill the diagonal.
+-   **k**: diagonal offset.
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   The order of the dimension indices contained in `dims` matters. The first element specifies the row-like dimension. The second element specifies the column-like dimension.
+-   Each provided dimension index must reside on the interval `[-ndims, ndims-1]`.
+-   The diagonal offset `k` is interpreted as `column - row`. Accordingly, when `k = 0`, the function fills the main diagonal; when `k > 0`, the function fills the diagonal above the main diagonal; and when `k < 0`, the function fills the diagonal below the main diagonal.
+-   If `value` is a number and `x` has a complex [data type][@stdlib/ndarray/dtypes], the function fills an input ndarray with a complex number whose real component equals the provided scalar `value` and whose imaginary component is zero.
+-   A `value` must be able to safely cast to the input ndarray [data type][@stdlib/ndarray/dtypes]. Scalar values having floating-point data types (both real and complex) are allowed to downcast to a lower precision data type of the same kind (e.g., a scalar double-precision floating-point number can be used to fill a `'float32'` input ndarray).
+-   The function **mutates** the input ndarray in-place.
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var zeros = require( '@stdlib/ndarray/zeros' );
+var ndarray2array = require( '@stdlib/ndarray/to-array' );
+var fillDiagonal = require( '@stdlib/ndarray/base/fill-diagonal' );
+
+// Create a stack of matrices:
+var x = zeros( [ 2, 3, 3 ] );
+
+// Fill main diagonals:
+fillDiagonal( x, 1.0, [ 1, 2 ], 0 );
+console.log( ndarray2array( x ) );
+
+// Fill super-diagonals:
+fillDiagonal( x, 2.0, [ 1, 2 ], 1 );
+console.log( ndarray2array( x ) );
+
+// Fill sub-diagonals:
+fillDiagonal( x, 3.0, [ 1, 2 ], -1 );
+console.log( ndarray2array( x ) );
+```
+
+</section>
+
+<!-- /.examples -->
+
+<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">
+
+[@stdlib/ndarray/dtypes]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/dtypes
+
+</section>
+
+<!-- /.links -->