feat: add C implementation for blas/ext/base/ndarray/csumkbn

[benatsf]

Refs #14819.

Description

This pull request adds a C API and optional Node.js native addon for @stdlib/blas/ext/base/ndarray/csumkbn, allowing compensated complex64 sums to run through the existing strided C kernel. The adapter converts ndarray byte strides and offsets into complex element indices and returns a Complex64 value. Browser and unavailable-addon environments retain the existing JavaScript implementation.

The complete package port includes its public header, N-API bridge, build manifests, C documentation/example/benchmark, and separate public, JavaScript, and native behavior tests.

Related Issues

• [RFC]: add C implementations for blas/ext/base/ndarray packages #14819

Questions

No.

Other

Validated locally with Node 24.19.0 and GCC: the addon builds and the public export selects it; 75/75 assertions pass with native tests running. Independent fixtures verify cancellation at 2^24 and 2^100, real/imaginary isolation, signed zero and NaN components, empty and singleton views, positive/negative/zero strides, buffer and ndarray offsets, and unchanged input/guard elements. Unavailable-addon dispatch selects the unchanged JS implementation. The C example returns 1+2i for a cancellation fixture; all 18 C benchmark runs complete. JavaScript, Markdown, package metadata, and both upstream EditorConfig configuration checks pass. Applicable C lint is recorded in the validation evidence.

Checklist

• Read, understood, and followed the contributing guidelines.

AI Assistance

• Yes

• No

• Code generation

• Test/benchmark generation

• Documentation (including examples)

• Research and understanding

Disclosure

The C adapter, Node.js addon, tests, build metadata, documentation, and research were prepared primarily by OpenAI Codex, following the project's existing ndarray/csum native implementation. The existing numerical C kernel and JavaScript implementation are reused. The contributing-guidelines acknowledgment records Codex's review and application of the guidelines under benatsf's explicit authorization to contribute. It does not claim that benatsf personally read the guide or manually reviewed the code. No manual applicant review is claimed.

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