fft-cnr: FFT-based contrast-to-noise ratio estimation from a single frame

Added

• Zenodo DOI badge and a Citation section with BibTeX in the README; the concept DOI and the verified method-foundation references in CITATION.cff.
• Prior-art acknowledgment (Foi et al., 2008) and a "Methods and background" paragraph citing the implemented method foundations (Welch 1967, Harris 1978, Akaike 1974, North 1963 / Turin 1960, Kay 1993, Janesick 2007 / EMVA 1288).

This release carries the citation metadata into the published artifact. Code is unchanged from 0.2.0, which was tagged and archived on Zenodo but not published to PyPI.

Full Changelog: https://github.com/thiebes/fft-cnr/compare/v0.2.0...v0.2.1