Add Kai et al. 2021 (Circ J) as a candidate reference PDF ...

Renal Dysfunction as a Predictor of Slow-Flow/No-Reflow Phenomenon
after PCI in STEMI (Circ J. 2021;85:1770-1778) — not yet cited in the
manuscript or added to references.bib; kept here for future use
(e.g. Related Work), per review discussed in chat.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>