feat: add stats/base/ndarray/meankbn2

[Bureir]

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• task: lint_filenames status: passed
• task: lint_editorconfig status: passed
• task: lint_markdown status: passed
• task: lint_package_json status: passed
• task: lint_repl_help status: passed
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Description

This PR adds @stdlib/stats/base/ndarray/meankbn2, an ndarray wrapper for computing
the arithmetic mean using a second-order Kahan–Babuška algorithm. The ndarray
implementation delegates to the strided meankbn2.ndarray kernel to follow project
conventions and avoid code duplication.

Changes include:

• lib/main.js: ndarray wrapper delegating to @stdlib/stats/strided/meankbn2.ndarray
• lib/index.js: module entry
• README.md: usage, notes, and example updated to meankbn2 with “second-order Kahan–Babuška” phrasing
• examples/index.js: demonstration script
• test/test.js: coverage for empty/stride/negative-stride/offset cases
• benchmark/benchmark.js: length sweep
• repl.txt: REPL help updated
• package.json: package name/description and directories
• docs/types (d.ts): type declarations aligned with API

Related Issues

None.

Questions

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None.

Other

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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

If you answered "yes" above, how did you use AI assistance?

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

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/ndarray/meankbn2	$\color{green}99/99$ $color{green}+100.00%$	$\color{green}3/3$ $color{green}+100.00%$	$\color{green}1/1$ $color{green}+100.00%$	$\color{green}99/99$ $color{green}+100.00%$

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

[Bureir]

@kgryte @headlessNode

[kgryte]

LGTM after minor clean-up.