Add CITATION.cff so GitHub shows "Cite this repository"

[RAFAELDCOELHO]

Summary

Adds a CITATION.cff file at the repo root so GitHub displays the "Cite this repository" widget. Docs-only change; no Python, tests, README, LICENSE, or Makefile touched.

Details

The file follows the same style as RAFAELDCOELHO/PersonaCore's CITATION.cff, with all metadata sourced from verifiable facts:

• license: MIT — matches the repository's GitHub SPDX license.
• date-released: 2026-07-28 — the repo's GitHub created_at date, which also matches the date of the v1.0 git tag.
• version: "1.0" — read from the existing v1.0 git tag (the README also states "Closed at v1.0").
• abstract — taken from the repo's public description; every claim is already in this repo's README.
• keywords — limited to what is true of this repo (from-scratch autograd, NumPy tensors, Transformer/GPT primitives, gradcheck, numerical parity against PyTorch).

No ORCID, DOI, identifiers, or preferred-citation were added. The YAML parses cleanly.