Function parse_version_constraint 

pub fn parse_version_constraint(version: Option<&str>) -> VersionConstraint

Parse a version string into a constraint.

• None / "latest" → Latest (any version, newest wins)
• "v1.2.3" → exact match
• "v2" → >=2.0.0, <3.0.0 (major range)
• "v2.1" → >=2.1.0, <2.2.0 (minor range)
• ">=0.5.0", "^2.0", "~1.2" → semver requirement
• anything else → branch/commit ref pin