feat: add blas/ext/base/sfirst-index-less-than-equal

[headlessNode]

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Resolves stdlib-js/metr-issue-tracker#1312.

Description

What is the purpose of this pull request?

This pull request:

• add blas/ext/base/sfirst-index-less-than-equal

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• Resolves [RFC]: add blas/ext/base/sfirst-index-less-than-equal metr-issue-tracker#1312.

Questions

Any questions for reviewers of this pull request?

No.

Other

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No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

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• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

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Primarily written by Claude Code.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
blas/ext/base/sfirst-index-less-than-equal	$\\color{green}372/372$ $\\color{green}+0.00\\%$	$\\color{green}19/19$ $\\color{green}+0.00\\%$	$\\color{green}4/4$ $\\color{green}+0.00\\%$	$\\color{green}372/372$ $\\color{green}+0.00\\%$

The above coverage report was generated for the changes in this PR.

[headlessNode]

While porting this package to dfirst-index-less-than-equal, a review surfaced a few consistency issues which are also present here:

1. docs/repl.txt: "which elements in the strided array are accessed" should be plural ("strided arrays"), as this is a two-array package.
2. test/test.sfirst_index_less_than_equal.js: the N <= 0 tape uses y = [ 2.0, 3.0, 4.0 ], while the native mirror and both ndarray test files use [ 1.0, 2.0, 3.0 ].
3. test/test.sfirst_index_less_than_equal.js: three // eslint-disable-line max-len comments (NaN/±0 tapes) are absent from the native mirror for identical-length lines.
4. src/addon.c: #include "stdlib/napi/export.h" is placed last among the NAPI includes; convention places it immediately after stdlib/blas/base/shared.h.
5. src/addon.c (addon_method): the STDLIB_NAPI_ARGV_STRIDED_FLOAT32ARRAY macro for X is interleaved between the INT64 macros; sibling packages (e.g., dfirst-index-equal) group all INT64 macros first (N, strideX, offsetX, strideY, offsetY), followed by X and Y.
6. README.md: eight <!-- eslint-disable id-length --> comments are unnecessary (sfirstIndexLessThanEqual is 24 characters, under the 25-character limit); only the max-len disable on the long .ndarray offsets example is needed.
7. docs/types/test.ts: two consecutive blank lines between the ndarray third-argument and fourth-argument $ExpectError blocks.
8. Main tapes: the first match is a strict-less hit (2 <= 5), so only the ±0 tape distinguishes <= from <; changing y[ 1 ] from 5.0 to 2.0 makes the first match an equality hit (2 <= 2) with the expected index unchanged.
9. ndarray test files: the stride/offset tapes place a blank line between the function call and t.strictEqual, while the main test files keep them adjacent with the blank line before t.end().