download: improve decide_checkout ref resolution

[soimkim]

Description

download: improve decide_checkout ref resolution

• Search over combined tag_set and branch_set (ref_set)
• Normalize base by stripping v/v./v for prefix-variant matching
• Add major.minor matching per Semver 2.0.0; support Name-X.Y.Z-style refs
• Prefer exact major.minor.patch match, then highest patch
• Priority: exact → prefix variant → major.minor → endswith

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation update
• Refactoring, Maintenance
• Breaking change (fix or feature that would cause existing functionality to not work as expected)