Address CI review P3: document citation-vs-package authorship split

Reviewer flagged that README BibTeX (Gerber, Isaac) and CITATION.cff now
diverge from pyproject.toml / docs/conf.py / layout.html, which still say
"diff-diff contributors." Of the reviewer's two suggested fixes (align all
surfaces, or document the intentional split), documenting is the right
move — package metadata using a collective name is a standard OSS
convention that acknowledges Wenli Xu and future contributors without
churning pyproject.toml on every new PR. Academic citation using
individual authors with ORCIDs is a separate scholarly convention.

Adds a brief "Note on authorship" after the BibTeX block in README.md's
Citing section, pointing readers to CONTRIBUTORS.md for the collective
list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

README.md

@@ -3080,6 +3080,8 @@ The DOI above is the Zenodo concept DOI — it always resolves to the latest rel
 
 See [`CITATION.cff`](CITATION.cff) for the full citation metadata.
 
+**Note on authorship**: academic citation (`CITATION.cff`, the BibTeX above) lists individual authors with ORCIDs per scholarly convention. Package metadata surfaces (`pyproject.toml`, Sphinx docs) list "diff-diff contributors" to acknowledge the collective — see [`CONTRIBUTORS.md`](CONTRIBUTORS.md) for the full list.
+
 ## License
 
 MIT License