Refine Introduction wording and add two more verified citations ...

- Add wang2026 (USCNet) and hibi2013 (IVUS history review) to
  references.bib after reading the full PDFs; cite them for KSCNet's
  originating kidney-stone-classification architecture and for the
  claim that IVUS can visualize vessel wall/plaque condition,
  respectively.
- Drop "microvascular spasm" as a contributing factor from the
  no-reflow definition, trim an editorializing clause after the
  attenuated-plaque finding, and rephrase the IVUS-focus sentence as
  the paper's own rationale rather than a general background claim.
- Remove the unsupported vasodilator-treatment-limitations sentence
  and the citation-needed marker on preventive-measures (distal
  protection devices/prophylactic vasodilators), per review: none of
  the added source papers support the former, and the latter is
  standard clinical practice not requiring a citation.
- Generalize the poor-prognosis sentence (previously citing specific,
  unsupported outcomes) to a claim not requiring a citation.

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