Clean up code reuse, consistency, and efficiency issues

Merge readExistingSerial and readExistingContent into a single
readExisting function to eliminate duplicate file I/O. Extract dateBase
helper to deduplicate serial formula between defaultSerial and
bumpSerial. Cache hash results during collision check to avoid
recomputing per member. Normalize error prefixes (remove "error:" from
fmt.Errorf, add uniformly at print sites). Use filepath.Join instead of
manual "/" concatenation. Replace trivial containsStr wrapper with
strings.Contains. Simplify tokenize to a single return. Use writeTestFile
and fixedTime helpers consistently in tests.

input.go

@@ -54,7 +54,7 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 	// Split on whitespace first, then handle comma separation within tokens
 	tokens := tokenize(line)
 	if len(tokens) < 2 {
-		return ZoneEntry{}, fmt.Errorf("error: %s:%d: expected zone name followed by at least one catalog name", file, lineNum)
+		return ZoneEntry{}, fmt.Errorf("%s:%d: expected zone name followed by at least one catalog name", file, lineNum)
 	}
 
 	entry := ZoneEntry{
@@ -69,16 +69,16 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 			switch strings.ToLower(key) {
 			case "group":
 				if value == "" {
-					return ZoneEntry{}, fmt.Errorf("error: %s:%d: empty group value", file, lineNum)
+					return ZoneEntry{}, fmt.Errorf("%s:%d: empty group value", file, lineNum)
 				}
 				entry.Group = value
 			case "coo":
 				if value == "" {
-					return ZoneEntry{}, fmt.Errorf("error: %s:%d: empty coo value", file, lineNum)
+					return ZoneEntry{}, fmt.Errorf("%s:%d: empty coo value", file, lineNum)
 				}
 				entry.COO = normalizeFQDN(value)
 			default:
-				return ZoneEntry{}, fmt.Errorf("error: %s:%d: unknown property %q", file, lineNum, key)
+				return ZoneEntry{}, fmt.Errorf("%s:%d: unknown property %q", file, lineNum, key)
 			}
 		} else {
 			// Bare name = catalog assignment
@@ -87,7 +87,7 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 	}
 
 	if len(entry.Catalogs) == 0 {
-		return ZoneEntry{}, fmt.Errorf("error: %s:%d: no catalog assignment for zone %s", file, lineNum, entry.Zone)
+		return ZoneEntry{}, fmt.Errorf("%s:%d: no catalog assignment for zone %s", file, lineNum, entry.Zone)
 	}
 
 	return entry, nil
@@ -95,10 +95,7 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 
 // tokenize splits a line on whitespace and commas, stripping empty tokens.
 func tokenize(line string) []string {
-	// Replace commas with spaces, then split on whitespace
-	line = strings.ReplaceAll(line, ",", " ")
-	fields := strings.Fields(line)
-	return fields
+	return strings.Fields(strings.ReplaceAll(line, ",", " "))
 }
 
 func buildCatalogMembers(entries []ZoneEntry, cfg *Config) (CatalogMembers, error) {
@@ -110,14 +107,14 @@ func buildCatalogMembers(entries []ZoneEntry, cfg *Config) (CatalogMembers, erro
 	for _, entry := range entries {
 		for _, catName := range entry.Catalogs {
 			if _, ok := cfg.Catalogs[catName]; !ok {
-				return nil, fmt.Errorf("error: %s:%d: unknown catalog %q", entry.File, entry.Line, catName)
+				return nil, fmt.Errorf("%s:%d: unknown catalog %q", entry.File, entry.Line, catName)
 			}
 
 			if seen[catName] == nil {
 				seen[catName] = make(map[string]int)
 			}
 			if prevLine, dup := seen[catName][entry.Zone]; dup {
-				return nil, fmt.Errorf("error: %s:%d: zone %s already assigned to catalog %q at line %d",
+				return nil, fmt.Errorf("%s:%d: zone %s already assigned to catalog %q at line %d",
 					entry.File, entry.Line, entry.Zone, catName, prevLine)
 			}
 			seen[catName][entry.Zone] = entry.Line