fix(semver): skip non-semver tags when picking latest existing tag

Rolling tags like 'v1' (and any other tag that doesn't parse as x.y.z
after prefix stripping) used to enter the latest-tag candidate set in
CalculateSemver. When such a tag shared a commit with a real semver tag
(e.g. v1 and v1.16.3 both pointing at the same release commit), go-git's
alphabetical tag iteration made 'v1' win, ParseExistingSemver bailed out
because '1' has only one component, and the calculator silently reset
the baseline to 0.0.0 — producing nonsense like 0.0.5 on this branch.

ListExistingTags now filters tags through a new IsParseableSemverTag
helper before recording them, so non-semver tags never participate in
latest-tag selection. The behavior change is invisible for repos that
only use proper vX.Y.Z tags, and it's covered by a new test table.

cmd/main.go

@@ -129,7 +129,7 @@ func main() {
 
 		// List existing tags if needed
 		if params.varExisting || repo.Config.Force.Existing {
-			utils.ListExistingTags(&repo.GitRepo)
+			utils.ListExistingTags(&repo.GitRepo, repo.Config.TagPrefixes)
 		}
 
 		// Apply forced versioning