Function is_major_minor_prefix 

pub fn is_major_minor_prefix(s: &str) -> bool

Return true when s is a kernel major.minor prefix like "6.14" (as opposed to a full patch version "6.14.2" or an rc tag "6.15-rc3"). Callers use this to decide whether the input needs prefix resolution via fetch_version_for_prefix.

Accepts any string with fewer than 2 dots and no -rc substring, so "7" (single-segment) and "" both return true. This matches the historical inline check used by kernel-build dispatchers.