blurSigma property
Gaussian sigma of the frost blur (logical px).
Applied as ImageFilter.blur composed before the glass shader.
Impeller blurs with a Gaussian of exactly this sigma, truncated at
radius round((σ − 0.5)·√3) physical px and renormalised (measured in
Task 15c). The truncation makes the effective blur 0.8–0.95× narrower
than the requested sigma (see docs/superpowers/notes/shader-probe.md);
fitting works with the requested value.
Implementation
final double blurSigma;